Opponents of Iran's most senior dissident cleric Grand Ayatollah Hossein Ali Montazeri stopped his memorial service in a tumultuous day in Qom Monday which saw huge protests and some shots fired, websites said. The car of Iranian opposition leader Mir Hossein Mousavi was attacked by “plainclothes men” on motorbikes as he was returning to Tehran from the service in Qom and one member of his entourage was injured, one of the websites said. The incidents came after huge crowds of Iranians marked Montazeri's funeral in Qom, 80 miles south of Tehran. Websites reported scuffles between mourners and police, with one saying warning shots were fired. “Around 2,000 of Montazeri's opponents arrived at Azam mosque and halted his memorial service. The assailants tore placards,” the conservative Ayande website said, referring to a prominent Qom mosque. As a result, “his memorial service was cancelled halfway through,” Ayande said, adding that security forces “did not take measures to prevent the assailants' act of sabotage.” Montazeri, who died during Saturday night aged 87, was an architect of the 1979 revolution that overthrew the US-backed shah and was once named to succeed Ayatollah Ruhollah Khomeini as supreme leader of the Islamic Republic. But Montazeri fell from grace after criticizing the mass execution of prisoners. An outspoken critic of Khomeini's successor as supreme leader, Ayatollah Ali Khamenei, Montazeri was viewed as the spiritual patron of the pro-reform opposition movement that led the big protests following June's disputed presidential polls. Despite dozens of arrests and security crackdowns, the protests have repeatedly flared up. The reports from Qom could not be verified independently because foreign media were banned from reporting directly on protests and were told not to travel to Qom for Montazeri's funeral, which took place earlier Monday. A reformist website, Kaleme, said Mousavi's car was attacked while he was returning to Tehran from the funeral and the car's back window was smashed in the attack. “On the way back from Qom to Tehran before noon ... a group of plainclothes men riding motorcycles attacked the car carrying Mousavi, as a consequence of which one member of Mousavi's entourage was injured,” Kaleme said. “One of the assailants shattered the back window of the car carrying Mousavi,” Kaleme said, adding that one of the attackers was also injured in the incident. Earlier, violence flared when security forces around Montazeri's house clashed with stone-throwing protesters, the reformist website Norooz said. There was no official comment. The moderate Parlemannews website cited “information received” of shots fired in the air near Qom's main shrine and also of the use of tear gas, without giving details. Pictures obtained by Reuters showed scuffles apparently between government and opposition supporters. The reformist website Jaras said hundreds of thousands of people joined a procession for Montazeri. “Innocent Montazeri, your path will be continued even if the dictator should rain bullets on our heads,” the crowd chanted. The Ayande website estimated the number of people attending the funeral ceremony at tens of thousands.