Iranian security forces fired tear gas and clashed with anti-government protesters demonstrating against the treatment of opposition leaders, pro-reform websites reported 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 tear gas 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,” it added, and said they demanded the release of Mir Hossein Mousavi and Mehdi Karroubi, under house arrest in Tehran since Feb. 14. Inspired by uprisings in the Middle East, thousands of their supporters have taken to the streets, defying a heavy security presence, 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 Tuesday. “Riot police attacked protesters with batons and electric shocks in Tehran,” Kaleme reported. Two people were killed and dozens arrested during a Feb. 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 quoted an “informed source” as saying Mousavi and Karroubi, along with their wives, had been moved from their homes to a “‘safe house’ in an area close to Tehran.” The judiciary denied reports Monday that the two had been jailed. Their arrest might ignite tension in the Islamic Republic, with hard-line rulers wary about any spillover of popular turmoil in the Arab world into Iran. British Foreign Secretary William Hague said he was deeply concerned by the reports that Mousavi and Karroubi had been moved. “I call on the Iranian authorities to release both men immediately,” Hague said, adding that calls by pro-government parliamentarians for the men to be executed were “inexcusable.” Hard-liners 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. 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. Mousavi and Karroubi say the reform movement is still alive despite mounting pressure on opposition supporters.