Iranian reformists have clashed with police after the funeral of a dissident cleric, opposition websites say, BBC reported.
Earlier, tens of thousands took part in a procession for Grand Ayatollah Hoseyn Ali Montazeri in the holy city of Qom.
Clashes reportedly broke out, but the scale of the confrontation is not clear, says a BBC correspondent.
Montazeri - who died aged 87 of natural causes in Qom on Saturday night - had decried President Mahmoud Ahmadinejad's re-election in June as a fraud.
The reformist Jaras website said mourners chanted slogans in support of the cleric and also of opposition leader Mir Hossein Mousavi.
Mr Mousavi took part in the procession, along with fellow opposition leader Mehdi Karroubi.
Another site, Rahesabz.net, said some members of the hardline pro-government faction Ansar Hezbollah tried to stop chanting in the crowd, but left "after clashing with some people".
Many mourners were carrying green banners or wearing green - the colour of Iran's opposition.
The ayatollah's son, Saeed Montazeri, said the security forces had surrounded his father's home after the ceremony in the holy city of Qom.
Opposition website Kaleme.org said that following the funeral, some mourners threw stones at police surrounding the cleric's house and clashes with security forces followed.
Footage broadcast on the internet showed crowds chanting against Iran's Supreme Leader Ayatollah Ali Khamenei, calling him a "murderer" and saying his rule was illegitimate.
Meanwhile, amateur footage sent to BBC Persian interactive apparently shows government supporters outside Montazeri's house, holding posters of the supreme leader.
Policemen wearing blue uniforms with NAJA on the backs, are holding them in line.
Other video showed thousands of mourners marching in Montazeri's home town of Najafabad, near the central city of Isfahan.
They beat their chests and chanted: "Oppressed Montazeri, you are with God now."
The government knows the funeral is one demonstration it cannot forbid, says the BBC's Tehran correspondent Jon Leyne, although it fears it could spark another round of protests.
In response, authorities have imposed strict new restrictions on the media, both foreign and domestic.
Leading up to the funeral, buses carrying mourners were stopped and some passengers reportedly arrested.
The Jaras website said one of Montazeri's students, Ahmad Qabel, had been detained on his way to the funeral.
Meanwhile, the BBC said fresh attempts have been made to jam its Persian television service to Iran.
Persistent interference started on Sunday shortly after the channel began coverage of the grand ayatollah's death, the corporation said.
BBC Persian TV suffered similar attacks during the Iranian presidential elections in June.
Mr Mousavi and Mr Karroubi were both presidential candidates in last June's disputed elections.
Anger at the result saw mass protests in Tehran and other cities that led to thousands of arrests and some deaths.
They have called for Monday to be a national day of mourning.
The grand ayatollah was one of Shia Islam's most respected figures.
He was an architect of the 1979 revolution that created Iran's Islamic state but later became a vocal critic of the government.
'Rioters' cleric'
BBC religious affairs correspondent Christopher Landau says that in a country where the younger generation is often seen as increasingly secularised, Montazeri offered an understanding of Islam in sympathy with many of the opposition's concerns.
His death deprives Iran of an outspoken champion of an alternative vision for the Islamic Revolution, our correspondent says.
The cleric's death also comes during the sacred first month of the Islamic calendar. The 10th day of that month, the Shia festival of Ashura, is this coming Sunday.
It was already likely to offer a platform for opposition protests, and this death gives that date added significance, our correspondent adds.
On 20 December, following news of the grand ayatollah's death, a large crowd quickly gathered outside his home in Qom to pay respects.
Sporadic gatherings were also reported in Tehran and other cities.
Foreign media have been restricted in Iran since the post-election protests and it is often difficult to verify reports. No foreign media have been allowed to attend the funeral.
Supreme Leader Ayatollah Ali Khamenei did offer his condolences on Sunday, praising Grand Ayatollah Montazeri as an outstanding jurist.
However, the Supreme Leader tempered this with criticism, saying he hoped God would forgive the late cleric for failing his "crucial test".
This was a reference to Montazeri's falling out with the leader of the revolution, Ayatollah Ruhollah Khomeini.
State news agency Irna did not use the grand ayatollah title in its early reports of Montazeri's death, and referred to him as the "rioters' cleric".
The White House praised Montazeri, saying he had been "known and internationally respected for his unwavering commitment to universal rights".
Hoseyn Ali Montazeri was one of the early backers of Ayatollah Khomeini and was designated to succeed him.
But the pair fell out over Iran's human rights record a few months before Khomeini died of cancer in 1989.
In 1997 he famously clashed with Khamenei, whom he outranked in the religious hierarchy, after questioning the powers of the supreme leader.
Montazeri repeatedly accused the country's rulers of imposing dictatorship in the name of Islam and said the liberation that was supposed to have followed the 1979 revolution never happened.
Then, despite his old age and failing health, Montazeri backed the opposition's claims that the 2009 election result, which gave Mahmoud Ahmadinejad a landslide victory, had been widely rigged.
Montazeri said Iran's leadership was a dictatorship and issued a fatwa condemning the government.