Amateur video said to be of Iranian opposition protesters in Najafabad
Iranian security forces have clashed with opposition supporters in the city of Isfahan, opposition websites say.
Activists said police used tear gas and batons to disperse people gathering to commemorate Grand Ayatollah Hoseyn Ali Montazeri, who died at the weekend.
Security forces reportedly surrounded the home of an ayatollah who organised the memorial service.
On Monday, tens of thousands of mourners attended Montazeri's funeral in the holy city of Qom.
Many of them shouted anti-government slogans.
The US has accused Iran of behaving like a "police state".
The funeral saw reports of clashes between security forces and mourners - with confrontations continuing in Qom on Tuesday.
State television reported that government supporters staged counter-demonstrations on Tuesday and Wednesday in Qom.
Born into provincial family in 1922 and educated at a seminary
Arrested and tortured for leading protests against Iran monarchy
Designated successor to Islamic Republic's founder, Khomeini
Fell out with Khomeini in 1989 over Iran's human rights record
House arrest in 1997 for criticising current Supreme Leader
Issues a fatwa against President Ahmadinejad after 2009's election
Reformists say there has also been unrest in Montazeri's home city of Najafabad over the past two days.
Footage sent to the BBC from Najafabad shows crowds chanting "Criminals, rapists, death to the leadership" and "We're not afraid, we're not afraid" as security men watch from the rooftops.
The sender says the footage, which has not been independently verified, was shot in the main mosque.
BBC Tehran correspondent Jon Leyne says the confrontations are all part of a build-up to a big series of demonstrations expected at the weekend.
He says that Isfahan and Najafabad are known as quite religiously conservative cities, which shows the breadth of the opposition to the government.
The authorities have not yet confirmed the unrest in Isfahan, but the country's police chief warned on Wednesday that opposition protests would not be tolerated.
"We advise this movement to end their activities," the semi-official Fars news agency quoted Esmail Ahmadi Moghaddam as saying.
"Otherwise those who violate the order will be fiercely confronted, based on the law."
However, our correspondent says that these threats do not seem to have much effect, because when people get beaten up it just angers them more and they still come out on the streets.
White House spokesman Philip Crowley said on Wednesday that Iran was "increasingly showing itself to be a police state".
He added: "It is using all of its levers, all of its various security elements to try to stamp out clearly the aspirations of the Iranian people.
"And yet the people keep on finding a way to exercise their universal rights of freedom of assembly, freedom of speech."
In Isfahan, witnesses told the BBC that people had gathered at the main mosque for the memorial service, but when they arrived the doors were closed and security forces told them to leave.
"Little by little some clashes broke out and security used tear gas and pepper gas," one witness said.
"They took people in the shops and beat them up mostly out of public vision although some beatings happened outside on the streets."
Another witness, who gave his name as Soheil, said security officers "beat people savagely" and did not care if the people were "women, men, old or young".
It had taken about two hours to disperse the crowd, he said.
The Rahesabz website said hundreds of police and plain-clothes security officers were involved.
Another reformist website, Parlemannews, said more than 50 people had been detained.
It said the home of Ayatollah Jalaleddin Taheri, who organised the memorial, had been surrounded by plainclothes security agents.
"I tried six different ways to get to the mosque but they were all blocked," Parlemannews quoted him as saying.
Reports are difficult to verify independently as foreign journalists have been restricted since the unrest that followed June's disputed presidential election.
The grand ayatollah's funeral in Qom was attended by several leading opposition figures, including Mir Hossein Mousavi.
Mr Mousavi, who came second in the presidential election, has been an outspoken critic of the current government and President Mahmoud Ahmadinejad.
On Tuesday, Mr Mousavi was dismissed as head of the Council for Cultural Revolution, an arts institution affiliated to the president's office.
In recent days, hardliners have urged Iran's judiciary to put Mr Mousavi on trial for instigating unrest.