A day after Saudi Arabia’s military rolled into Bahrain, the Iranian government branded the move “unacceptable” on Tuesday, threatening to escalate a local political conflict into a regional showdown with Iran.

“The presence of foreign forces and interference in Bahrain’s internal affairs is unacceptable and will further complicate the issue,” Ramin Mehmanparast, the Iranian Foreign Ministry spokesman told a news conference in Tehran, according to state-run media.

Even as predominantly Shiite Muslim Iran pursues a determined crackdown against dissent at home, Tehran has supported the protests led by the Shiite majority in Bahrain.

“People have some legitimate demands and they are expressing them peacefully,” Mr. Memanparast said. “It should not be responded to violently.”

“We expect their demands be fulfilled through correct means,” Mr. Mehmanparast added. Iran’s response — while anticipated — showed the depth of rivalry across the Persian Gulf in a contest that has far-reaching consequences in many parts of the Middle East.

On Monday, Iranian state-run media went so far as to call the troop movement an invasion. Saudi Arabia has been watching uneasily as Bahrain’s Shiite majority has staged weeks of protests against a Sunni monarchy, fearing that if the protesters prevailed, Iran, Saudi Arabia’s bitter regional rival, could expand its influence and inspire unrest elsewhere.

The Saudi decision to send in troops on Monday could further inflame the conflict and transform this teardrop of a nation in the Persian Gulf into the Middle East’s next proxy battlefield between regional and global powers. On Tuesday, there was no immediate indication that the Saudi forces were confronting protesters in the central Pearl Square — the emblem of the Bahrain protest much as Cairo’s Tahrir Square assumed symbolic significance in the Egyptian uprising.

Several hundred protesters camped out there on what seemed initially to be a quiet day with little traffic on the streets as the details of the deployment by Bahrain’s neighbors — and their mission — remained ill-defined.

On Monday, about 2,000 troops — 1,200 from Saudi Arabia and 800 from the United Arab Emirates — entered Bahrain as part of a force operating under the aegis of the Gulf Cooperation Council, a six-nation regional coalition of Sunni rulers that has grown increasingly anxious over the sustained challenge to Bahrain’s king, Hamad bin Isa al-Khalifa. “This is the initial phase,” a Saudi official said. “Bahrain will get whatever assistance it needs. It’s open-ended.”

The decision is the first time the council has used collective military action to help suppress a popular revolt — in this case a Shiite popular revolt. It was rejected by the opposition, and by Iran, as an “occupation.” Iran has long claimed that Bahrain is historically part of Iran.

The troops entered Bahrain at an especially combustible moment in the standoff between protesters and the monarchy. In recent days protesters have begun to move from the encampment in Pearl Square, the symbolic center of the nation, to the actual seat of power and influence, the Royal Court and the financial district. As the troops moved in, protesters controlled the main highway and said they were determined not to leave.

“We don’t know what is going to happen,” Jassim Hussein Ali, a member of the opposition Wefaq party and a former member of Parliament, said in a phone interview. “Bahrain is heading toward major problems, anarchy. This is an occupation, and this is not welcome.”

Rasool Nafisi, an academic and Iran expert based in Virginia, said: “Now that the Saudis have gone in, they may spur a similar reaction from Iran, and Bahrain becomes a battleground between Saudi and Iran. This may prolong the conflict rather than put an end to it, and make it an international event rather than a local uprising.”

An adviser to the United States government, who spoke on the condition of anonymity because he was not authorized to talk to the press, agreed. “Iran’s preference was not to get engaged because the flow of events was in their direction,” he said. “If the Saudi intervention changes the calculus, they will be more aggressive.”

Though Bahrain said it had invited the force, the Saudi presence highlights the degree to which the kingdom has become concerned over Iran’s growing regional influence, and demonstrates that the Saudi monarchy has drawn the line at its back door. Oil-rich Saudi Arabia, a close ally of Washington, has traditionally preferred to operate in the shadows through checkbook diplomacy. It has long provided an economic lifeline to Bahrain.

But it now finds itself largely standing alone to face Iran since its most important ally in that fight, Hosni Mubarak of Egypt, has been ousted in a popular uprising. Iran’s ally, Hezbollah, recently toppled the Saudi-backed government of Lebanon — a symbol of its regional might and Saudi Arabia’s diminishing clout.

But Bahrain is right at Saudi Arabia’s eastern border, where the kingdoms are connected by a causeway.

The Gulf Cooperation Council was clearly alarmed at the prospect of a Shiite political victory in Bahrain, fearing that it would inspire restive Shiite populations in Saudi Arabia and Kuwait to protest as well. The majority of the population in Saudi Arabia’s eastern provinces, where the oil is found, is Shiite, and there have already been small protests there.

“If the opposition in Bahrain wins, then Saudi loses,” said Mustafa el-Labbad, director of Al Sharq Center for Regional and Strategic Studies in Cairo. “In this regional context, the decision to move troops into Bahrain is not to help the monarchy of Bahrain, but to help Saudi Arabia itself .”

The Bahrain government said that it had invited the force in to help restore and preserve public order. The United States — which has continued to back the monarchy — said Monday that the move was not an occupation. The United States has long been allied with Bahrain’s royal family and has based the Navy’s Fifth Fleet in Bahrain for many years.

Though the United States eventually sided with the demonstrators in Egypt, in Bahrain it has instead supported the leadership while calling for restraint and democratic change. The Saudi official said the United States was informed Sunday that the Saudi troops would enter Bahrain on Monday.

Saudi and council officials said the military forces would not engage with the demonstrators, but would protect infrastructure, government offices and industries, even though the protests had largely been peaceful. The mobilization would allow Bahrain to free up its own police and military forces to deal with the demonstrators, the officials said.

The Gulf Cooperation Council “forces are not there to kill people,” said a Saudi official, speaking on the condition of anonymity because he was not authorized to talk to the press. “This is a G.C.C. decision; we do not violate international law.”

But the officials also acknowledged that it was a message to Iran. “There is no doubt Iran is involved,” said the official, though no proof has been offered that Iran has had anything to do with the political unrest.

Political analysts said that it was likely that the United States did not object to the deployment in part because it, too, saw a weakened monarchy as a net benefit to Iran at a time when the United States wants to move troops out of Iraq, where Iran has already established an influence.

The military force is one part of a Gulf Cooperation Council effort to try to contain the crisis in Bahrain that broke out Feb. 14, when young people called for a Day of Rage, fashioned after events in Egypt and Tunisia. The police and then the army killed seven demonstrators, leading Washington to press Bahrain to remove its forces from the street.

The royal family allowed thousands of demonstrators to camp at Pearl Square. It freed some political prisoners, allowed an exiled opposition leader to return and reshuffled the cabinet. And it called for a national dialogue.

But the concessions — after the killings — seemed to embolden a movement that went from calling for a true constitutional monarchy to demanding the downfall of the monarchy. The monarchy has said it will consider instituting a fairly elected Parliament, but it insisted that the first step would be opening a national dialogue — a position the opposition has rejected, though it was unclear whether the protesters were speaking with one voice.

The council moved troops in after deciding earlier to help prop up the king with a contribution of $10 billion over 10 years, and said that it might increase that figure. But if the goal was to intimidate Iran, or the protesters, that clearly was not the first response.

Bahrain’s opposition groups issued a statement: “We consider the entry of any soldier or military machinery into the Kingdom of Bahrain’s air, sea or land territories a blatant occupation.”

U.S. efforts to stabilize Bahrain, another key Arab ally roiled in popular uprising, is being threatened on several fronts—including apparent splits in Bahrain’s royal family and a sense of disengagement by Saudi Arabia, the region’s biggest power.

Whether the U.S. can halt the unrest in Bahrain is viewed as critical to stabilizing the Persian Gulf and checking Iran’s influence. But there is growing uncertainty in Washington over who in the tiny Middle East sheikdom’s royal family ordered the use of increasing force against unarmed protesters, according to officials briefed on the diplomacy.

Successive U.S. administrations have cultivated closed relations with Bahrain’s King Hamad bin Isa al-Khalifa and his son, the crown prince, both of whom are viewed as modernizers. But the island-state’s security forces are under control of the king’s uncle, Prince Khalifa bin Salman al-Khalifa, who also serves as prime minister.

Previous U.S. administrations have sought to convince King Hamad to remove his 76-year-old relative, according to former U.S. officials, following charges of corruption and his opposition to political liberalization. These officials said there is a growing likelihood Prince Khalifa is overseeing the crackdown, with the king and crown prince relegated to the sidelines.

“Our influence is largely with one part of the ruling family, and not with the prime minister,” said an American official in close contact with Bahrain’s government.

The situation in Bahrain is complicated by U.S. uncertainty over Saudi Arabia’s position on the growing regional turmoil. Riyadh has enormous influence over Bahrain’s royal family due to the financial and energy aid it provides. Riyadh has in the past sent its own security forces into Bahrain to quell unrest, concerned that Bahrain’s Shiite majority could fuel instability inside Saudi Arabia.

Still, Saudi Arabia’s King Abdullah and many of his closest advisers have been in Morocco in recent weeks as the Saudi monarch recovers from surgery. That has been seen as limiting the ability of other Saudi royals to make decisions. Other leading members of the Saudi royal family are also said to be in decline physically, particularly the second-in-line, Crown Prince Sultan, who is believed to be suffering from Alzheimer’s disease.

A spokesman for the Saudi Embassy in Washington didn’t respond to requests for comment.

Saudi officials voiced disapproval of the Obama administration’s handling of Egypt, in particular its decision to pull its support for President Hosni Mubarak, according to Arab diplomats. There has been little high-level contact between the U.S. and Saudi Arabia in recent weeks, U.S. officials said.

“There’s a leadership vacuum in Saudi Arabia, which is clouding the decision-making process,” said Simon Henderson, who tracks Saudi politics at the Washington Institute for Near East Policy.

Washington’s strategic alliance with Saudi Arabia has faltered in other theaters in the Middle East as well this year.

Last month, the militant Lebanese group Hezbollah overthrew the U.S.- and Saudi-backed government in Beirut, greatly enhancing Iran’s and Syria’s influence in the Mediterranean nation. Successive U.S. administrations had since 2005 worked with Riyadh to try and bolster former Lebanese Prime Minister Saad Hariri as a counterweight to Hezbollah’s backers in Tehran and Damascus. But Saudi Arabia ultimately pulled out of mediating efforts on behalf of Mr. Hariri, as Hezbollah threatened to sow unrest.

The fate of Bahrain, as Egypt before it, is crucial to U.S. strategic interests, and the unrest is showing up the administration’s inability to influence the course of events. Bahrain has been a crucial partner in Washington’s efforts to combat nearby Iran as well as al Qaeda. It sits in a key strategic position in the Persian Gulf and hosts the headquarters of the U.S. Fifth Fleet—a home to 3,000 military personnel who oversee the 30 naval ships and some 30,000 sailors.

“If the U.S. loses Bahrain, they risk losing the Persian Gulf,” said a senior Arab diplomat Friday.

President Barack Obama made his strongest call Friday for an end to the violence in Bahrain, urging the government “to show restraint in responding to peaceful protests, and to respect the rights of their people.”

King Khalifa announced Friday he had appointed his son, Crown Prince Salman bin Hamad al-Khalifa, to engage in a dialogue with the opposition once calm had returned. U.S. officials in Washington acknowledged this likely wouldn’t quell the protesters.

Officials wouldn’t comment on whether the Obama administration was directly seeking the removal of Prime Minister Khalifa. But the official said the prime minister “is hated by the majority in Bahrain.”

The prime minister, on his website, says he has supported the king’s reforms, specifically his press reforms.

Sen. Patrick Leahy (D., Vt.), chairman of a Senate panel that oversees foreign aid, has asked Secretary of State Hillary Clinton to review whether units of the Bahrain’s security forces used lethal force against civilians in violation of U.S. law, a finding that could prompt the U.S. to freeze assistance to those units.

The law, known as the Leahy Amendment, requires the U.S. to cut off aid to foreign security forces that are found to have committed gross human rights violations, and could provide a point of leverage for the Obama administration.