Iran, the center of Shiite Islam which has sometimes called Bahrain one of its own provinces, objected angrily to the troops’ arrival. The state media called it an invasion and the Foreign Ministry spokesman told a news conference in Tehran that the presence of foreign troops in Bahrain was “unacceptable.”

There is little evidence that the Shiite-led protests here have an Iranian sponsor or flavor. In fact, they are at least as much about demands for a democratic government as about sectarianism. But the risk of Iranian interference is clearly on the minds of the Saudis and the Americans.

Bahrain, a longtime American ally, is home to the United States Navy’s Fifth Fleet, which patrols the Persian Gulf and helps support the war in Afghanistan. The kingdom also allows American military aircraft to operate from its main air base.

But the Obama administration has been urging the royal family to step up long-promised political reforms. Last Saturday, Secretary of Defense Robert M. Gates stopped here to tell King Hamad bin Isa al-Khalifa that “baby steps” toward change were not enough. Jeffrey D. Feltman, an assistant secretary of state, was in Manama on Tuesday seeking solutions to what an American Embassy statement called “rising tensions and increased incidents of violence in Bahrain.”

A White House spokesman called for “calm and restraint on all sides.” He added: “We are particularly concerned by the increasing reports of provocative acts and sectarian violence by all groups. The use of force and violence from any source will only worsen the situation. One thing is clear: there is no military solution to the problems in Bahrain. A political solution is necessary and all sides must now work to produce a dialogue that addresses the needs of all of Bahrain’s citizens.”

Shortly after arriving in Cairo on Tuesday, Secretary of State Hillary Rodham Clinton telephoned Saudi Arabia’s foreign minister, Prince Saud al-Faisal, to express concern about the intervention in Bahrain. At a news conference, she said, “The use of force and violence from any source will only worsen the situation.” And she repeated the American call for a negotiated resolution between protesters and Bahrain’s government that seems more remote than ever after a declaration of a national emergency.