Opponents of Iran's most senior dissident cleric Grand Ayatollah Hossein Ali Montazeri stopped his memorial service in a tumultuous day in Qom which saw huge protests and some shots fired, websites said.

The car of Iranian opposition leader Mirhossein 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 the holy city, 125 km (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".

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 on 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.

Clashes near Montazeri's house

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.

Iran's internal unrest, highlighted by Montazeri's arguments that the leadership had lost its legitimacy, has complicated the dispute over the Iranian nuclear programme, which the West believes may have military ends, not just civilian purposes.

Opposition leaders Mousavi and Mehdi Karoubi were photographed paying their condolences at Montazeri's house.

Reformist websites said security forces arrested some opposition supporters trying to reach Qom and turned others away.

The cleric's death occurred in the tense run-up to Ashura, a politically laden Shi'ite religious commemoration that offers the opposition another opportunity to show its strength.

Shouts of "Oh Hossein, Mirhossein" also rose from mourners near Iran's second holiest shrine, many wearing green wristbands to show support for Mousavi.

Their cries echoed traditional Ashura laments for Hossein, a grandson of the Prophet Mohammad, killed in a 7th-century battle that sealed the schism between Sunni and Shi'ite Muslims.

Ashura, a key occasion in the Islamic Republic's calendar, will coincide with the seventh day of mourning for Montazeri, making it harder for authorities to keep people off the streets.

Barbed condolences

Mourners carried pictures of Montazeri and of another Qom-based dissident cleric, Grand Ayatollah Yusuf Sanei.

Supreme Leader Khamenei expressed condolences, but asked God to forgive Montazeri over a "difficult and critical test" that he faced towards the end of Khomeini's life -- making clear that he believed his old rival had failed the test.

Iranian newspapers published no pictures of Montazeri on their front pages, in line with what reformist websites said were orders from the Ministry of Culture and Islamic Guidance.

Khomeini's grandson, Hassan Khomeini, a cleric, paid tribute to a man who had "spent many years of his honourable life ... advancing the high goals of Islam and the Islamic revolution".

Human rights activist and Nobel Prize laureate Shirin Ebadi called Montazeri "the father of human rights in Iran".

President Mahmoud Ahmadinejad's re-election in a June vote that losing candidates Mousavi and Karoubi said was rigged kindled the biggest unrest in the Islamic Republic's 30-year history and split the political and clerical establishment. – Reuters