TEHRAN (Reuters) A pro-reform Iranian clerical group said on Sunday the outcome of last month's presidential vote was "invalid," even though Supreme Leader Ayatollah Ali Khamenei has upheld the result. In a sign of a deepening rift among Shi'ite clerics, the Assembly of Qom Seminary Scholars and Researchers also called for the release of Iranians arrested in protests after the hardline president was declared winner of the June 12 vote. "Other candidates' complaints and strong evidence of vote-rigging were ignored ... peaceful protests by Iranians were violently oppressed ... dozens of Iranians were killed and hundreds were illegally arrested," said a statement published on the Assembly's website. "The outcome is invalid." Qom is Iran's center of Shi'ite learning, about 80 miles south of Tehran. The assembly has little political influence but its statement is a significant act of defiance since Qom is the power base of the clerical establishment. It follows calls by hardliners for leaders of the protests, in which at least 20 people were killed, to be put on trial. The demonstrations were the most striking display of dissent in Iran since the 1979 Islamic revolution and the elite Revolutionary Guards on Sunday accused opposition leaders of "trying to overthrow the Islamic establishment." "We had forecast a velvet revolution. But it was neutralized by our vigilance," the official IRNA news agency quoted General Yadollah Javani as saying. Iran's police chief said on Wednesday 1,032 people had been detained during the protests in Tehran but most had been freed. Human Rights activists say as many as 2,000 people, including opposition leaders, professors, journalists and students, may be still be held across the country.