Iranian authorities launched a provocative attack on the opposition movement Tuesday by announcing a special investigation into prominent cleric Mehdi Karroubi over his accusations that security forces raped and tortured protesters following the disputed June re-election of President Mahmoud Ahmadinejad. The move against Karroubi, a revered figure from Iran's 1979 Islamic revolution, is an attack on the heart of the opposition. It's an indication that the government is increasing pressure on top dissenters, even clerics, and it follows the death sentences handed down in recent days against at least two anti-government protesters. The investigation will test the resolve of the opposition and has the potential to unleash another round of street demonstrations that in recent weeks have been largely thwarted by the Revolutionary Guard and the Basiji militia. At a rally in September, protesters shouted: 'If Karroubi is arrested, there will be insurrections across Iran.' The official Iranian news agency, IRNA, reported that 'a legal case prepared by the judicial investigative committee has been sent to the Tehran prosecutor over Karroubi's claims.' Chief Prosecutor Abbas Jafari-Dolatabadi was quoted by the agency as saying: 'Karroubi is a cleric, and his remarks should be studied at the special court for the clergy. Some people have also been summoned in connection with the case.' The 72-year-old cleric, whose offices were recently raided by government authorities who seized documents and computer files, has charged that security forces and prison guards raped detained protesters. The government has denied the claims, and Ahmadinejad and hard-line Republican Guard commanders have repeatedly called for his arrest. A judiciary panel found that Karroubi's 'allegations are unsubstantiated, and documents submitted are totally fabricated and aimed at misleading public opinion.' Karroubi, a former speaker of the Iranian parliament, finished fourth in the June presidential elections. He has aligned himself with opposition leader Mir Hossein Mousavi and former President Mohammad Khatami in calling for an end to police intimidation against demonstrators and in accusing Ahmadinejad of winning a fraudulent election. At least 30 people have been killed in postelection unrest, and more than 100 protesters are on trial before the nation's Revolutionary Court. (Begin optional trim) If Karroubi is accused by a clerical court of 'spreading false news and slanders against the regime, the rift between the people and the regime will widen and many other officials may be tried,' said Nemat Ahmadi, a human rights lawyer. 'Karroubi is not simply a person. He was twice speaker of parliament, and he was close to the late Ayatollah Khomeini in the pre- and post-revolution Iran. I hope this is not more than just a threat against him and doesn't lead to a trial in clerical court.' Mohammad Siefzadeh, a human rights lawyer who recently has been forbidden to leave Iran, said: 'I do not know what will happen to Mr. Karroubi, but I know if he cannot prove his assertions (of torture) by concrete evidence, he will not be given an exception, no matter how well-established a politician, cleric and revolutionary he is.' In an interview with the Los Angeles Times in September, Karroubi, whose newspaper was shut down weeks ago, said: 'I won't go underground. I act publicly and openly. Even if I am arrested and jailed and released, I will go back to open activities.' (End optional trim) The investigation into Karroubi comes days after his ally Khatami posted on his Web site a condemnation of the government's repression of protest and a larger criticism that Ahmadinejad and Supreme Leader Ayatollah Ali Khamenei are betraying the ideals of the Islamic revolution. 'Be sure that people will never back down,' said Khatami, who was president from 1997 to 2005 and has become a leading reformist voice. 'Today, we are living in a world in which no dictator could be imposed on people to force them to be absolutely obedient to him. An acceptable government is a government born out of the people. 'Why are we presenting Islam in a way to encourage segments of the society to turn their back on religion and the regime?' Times staff writer Fleishman reported from Cairo, Egypt, and special correspondent Mostaghim from Tehran. 