CAIRO — Tens of thousands of protesters faced club-wielding security forces Monday in Bahrain, Yemen and Iran in what experts said may be shaping up as a pro-democracy wave ignited by the revolts that drove Egypt and Tunisia's autocratic rulers from power.

At least one protester was confirmed killed in Bahrain, and there were unconfirmed reports of several deaths and hundreds of arrests in Tehran, where anti-government marchers chanting "death to dictators" staged their largest demonstration in more than a year.

The Obama administration spoke out for the Iranian protesters, in contrast to relatively mild initial statements it issued when unrest erupted there after disputed presidential elections in 2009.

"Let me very clearly and directly support the aspirations of the people who are in the streets in Iran today," Secretary of State Hillary Clinton said after a Capitol Hill meeting with House Speaker John Boehner, R-Ohio.

"What we see happening in Iran today is a testament to the courage of the Iranian people and an indictment of the hypocrisy of the Iranian regime, a regime which over the last three weeks has constantly hailed what went on in Egypt," Clinton said.

The turnout in Tehran was especially surprising, given the executions and other harsh penalties meted out to people arrested in protests there after disputed June 2009 elections.

News reports put the crowds at tens of thousands, the largest since December 2009, when Iranian authorities crushed protests. Press TV, the state-run English-language television channel, said small groups of protesters "disrupted order in the Iranian capital Tehran," provoking counterprotests by government supporters.

Cell-phone videos posted on Twitter, YouTube and other social networking sites showed crowds chanting anti-regime slogans.

"The turnout exceeded my expectations," said Karim Sadjadpour, an Iran specialist at the Carnegie Endowment for International Peace. Still, he said, "Many people don't see the opposition has a clear strategy or end game in mind and hence are unwilling to take to the streets and risk their lives for ambiguous goals."

In Bahrain, a Manhattan-size emirate with a population of 738,000, security forces attacked peaceful protesters to prevent them from massing in Manama, the capital, according to news reports and Internet posts.

Twitter feeds spoke of police firing tear gas, bird shot, rubber bullets and live ammunition. A YouTube video showed dozens of people protesting peacefully at a traffic circle, some waving red and white Bahraini flags. Suddenly, blue-suited police charge from the opposite side of the circle as tear gas grenades explode near the protesters.

The emirate's interior minister later expressed condolences to the family of the dead protester, identified as Ali Abdulhadi Mushaimi, 27, and pledged to investigate the killing and whether whoever fired the fatal shot had acted improperly, the official Bahrain News Agency said.

A purported photograph of the dead man was posted on the Internet, along with photographs of expended U.S.-made teargas canisters. Bahrain is the headquarters of the U.S. 5th Fleet.

In Yemen, security forces and pro-regime groups clashed for the fourth straight day with university students, journalists and others demanding democratic reforms and the ouster of President Ali Abdullah Saleh, who has ruled the poor nation at the tip of the Arabian peninsula for 32 years.

New York-based Human Rights Watch said protesters were attacked "apparently without justification."

Nasser Arrabyee, a freelance journalist and blogger, reported on his website that opposition parties withdrew an agreement to open a dialogue with Saleh after the unrest, which was centered at Sanaa University in the capital.

U.S. officials are deeply concerned about Yemen, where an affiliate of al-Qaida has been making inroads. Saleh also faces an uprising in the north by a Shiite sect and a secessionist movement in the south, which was independent until 1990.

There was also some minor violence in Egypt, where several hundred state employees, including police and transport workers, demonstrated to demand better pay and working conditions. The country's new military rulers urged an end to strikes because they were "harming national security."

In Cairo, about 200 workers protested outside the state-controlled Egyptian Federation of Trade Unions to demand the removal of the union boss, Hussein El Magawer, a wealthy businessman and member of former President Hosni Mubarak's National Democratic Party.

Witnesses said the workers attempted to enter the building peacefully but were met with force by government-allied federation officials, who threw glass bottles, sticks and a fire extinguisher at the demonstrators.

The scuffle, which injured three, went on for about 30 minutes before Egyptian military police intervened and arrested one of the federation officials.