Iran's top dissident cleric Grand Ayatollah Hossein Ali Montazeri, a fierce critic of the hardline leadership who denounced June's disputed presidential election as fraudulent, has died at the age of 87.

The moderate 'Parlemannews' website said supporters of Montazeri, an architect of the 1979 Islamic revolution, were flocking to the holy city of Qom to attend his funeral on Monday.

The reformist 'Tagheer' website also reported that opposition supporters were gathering in squares of Tehran on Sunday to mourn his death.

Montazeri died from a heart attack, official media reported on Sunday, and one analyst said his burial may turn into an opposition show of strength.

His death on Saturday coincides with tension rising once again in the country, six months after the presidential poll plunged the major oil producer into political crisis.

'My grandfather died in his sleep last night. People and friends are coming to express their condolences but there are no special security measures around our house,' Naser Montazeri told Reuters by phone from Qom, about 125 km south of Tehran.

Monday's burial, to start at 9am, could become a rallying point for the reformist opposition and this may worry the authorities, London-based Iran analyst Baqer Moin said.

'The amount of support shown to him will hearten the opposition who are mourning his loss,' Moin said.

A moderates' website said Montazeri followers were travelling from other parts of Iran to Qom, a Shi'ite Muslim religious centre. 'Thousands of people from Isfahan, Najafabad and other cities are going to Qom to attend Montazeri's funeral on Monday,' Parlemannews said.

The Tagheer website of pro-reform cleric Mehdi Karoubi reported that Montazeri followers were gathering in Tehran.

'The social network of the reform movement has called on its supporters to gather in Mohseni square to mourn ... based on reports people have already gathered in some other squares in Tehran,' it said.

Montazeri was named in the 1980s to succeed revolutionary leader Ayatollah Ruhollah Khomeini as Iran's top authority, but fell out with him over the mass execution of prisoners.

One of Iran's most senior clerics, he spent five years under house arrest until 2002 but remained a leading opposition voice until his death, even though he rarely left his home in Qom.

'He will be remembered as a man who sacrificed his political position for the sake of his principles,' said Moin, describing him as an inspiration for other pro-reform clerics.

Montazeri, who was a close ally of Khomeini before the revolution and jailed several times by the US-backed Shah's police, was among the government's harshest critics in a clerical establishment whose splits have widened during the turmoil triggered by June 12 vote.

In August, the ayatollah said on his web site that authorities' handling of street unrest following the election 'could lead to the fall of the regime' and he denounced the clerical leadership as a dictatorship.

The pro-reform opposition says the poll was rigged to secure hardline President Mahmoud Ahmadinejad's re-election.

The authorities have denied the charge and portrayed the huge opposition protests after the election, which were quelled by the elite Revolutionary Guards and Islamic militiamen, as a foreign-backed bid to undermine the clerical leadership.

Tension increased earlier this month when pro-opposition students clashed with the security forces armed with batons and tear gas in the biggest anti-government protest in months.

Iran's judiciary last week threatened legal action against senior opposition figures for fomenting post-vote unrest.

But pro-reform cleric Karoubi, who came fourth in the election, said threats of arrest would only make him and others more determined to stick to their path.

'They think that people's reform movement depends on one person ... I am determined to continue this path and with threats of arrests I will not step down,' Karoubi said in a statement on his website on Sunday.

Official media initially did not give prominent coverage to Montazeri's death, but it topped state television's main afternoon broadcast.

The official IRNA news agency said 'problem elements' in Montazeri's household and his statements 'appreciated by enemies of the Islamic Republic' were to blame for his estrangement with Khomeini two decades ago.

Instead of Montazeri succeeding Khomeini upon the death of the Islamic Republic's founder in 1989, current Supreme Leader Ayatollah Ali Khamenei became the country's top authority.-Reuters