TEHRAN (Reuters) - Iranian security forces fired teargas and clashed with anti-government protesters demonstrating against the treatment of opposition leaders, pro-reform websites reported on Tuesday.
Thousands of demonstrators poured into the streets of Tehran and other cities, chanting slogans against the government, Sahamnews reported.
"Security forces and plainclothes agents fired teargas and clashed with demonstrators in Tehran to disperse them," another opposition website Kaleme reported.
"Protesters have formed groups in hundreds and are marching toward Tehran's Azadi (Freedom) Square."
Potesters demanded the release of Mirhossein Mousavi and Mehdi Karoubi, forced to stay in their homes in Tehran since February 14. In the meantime, thousands of their supporters have taken to the streets, defying a heavy security presence, to back uprisings in Egypt and Tunisia, Sahamnews said.
Seeking to avoid a revival of mass anti-government rallies that erupted after a disputed 2009 presidential election, the authorities had warned against any "illegal" gatherings after some opposition websites posted calls for a rally on Tuesday.
"Riot police attacked protesters with batons and electric shocks in Tehran," Kaleme reported.
Two people were killed and dozens arrested during a February 14 rally, the first big show of opposition since the elite Revolutionary Guards crushed street protests in December 2009.
The International Campaign for Human Rights in Iran, which has staff in the United States and Germany, quoted an "informed source" as saying Mousavi and Karoubi, along with their wives, had been moved from their homes to a "'safe house' in an area close to Tehran."
The judiciary denied reports on Monday that the two had been jailed. Their arrest might ignite tension in the Islamic Republic with hardline rulers wary about any spillover of popular turmoil in the Arab world into Iran.
In London, British Foreign Secretary William Hague said in a statement he was deeply concerned by reports that Mousavi and Karoubi had been moved from their houses to an Iranian detention center.
"I call on the Iranian authorities to release both men immediately," Hague said, adding that calls by pro-government Iranian parliamentarians for the opposition leaders to be executed were "inexcusable."
Sahamnews said "tension was increasingly growing" in Tehran and some other cities, adding that large numbers of security personnel were stationed at main streets and squares in Tehran "to prevent gathering of opposition supporters."
"Gunshots were heard around Enqelab (Revolution) Square and nearby streets," Kaleme reported.
Hardliners have urged the judiciary to hand down death penalties to opposition leaders, accusing them of being part of a Western plot to overthrow the Islamic system.
Mousavi and Karoubi, who spearheaded protests against President Mahmoud Ahmadinejad's re-election in 2009, say the reform movement is still alive despite mounting pressure on opposition supporters.
editing by Paul Taylor