Iranian police have fired tear gas to disperse opposition supporters mounting protests in the capital Tehran.
A BBC correspondent in Tehran said large numbers of riot police and militia on motorcycles in the city centre broke up any crowds that formed.
The unrest comes a day after websites close to opposition leaders Mir Hossein Mousavi and Mehdi Karroubi said they had been jailed.
The government has denied this, saying the two men were at home.
The BBC's Mohsen Agsari in Tehran said by early evening the security forces appeared to have full control of the streets.
The Basiji militia were chanting victory slogans, he said.
Earlier, despite the presence of security forces in Tehran's main streets and squares, sporadic gatherings were held close to the main Azadi square, our correspondent said.
Demonstrators chanting "Ya Hossein, Mir Hossein" were immediately attacked by the police.
After night fell, demonstrators made random protests, using the cover of darkness to confuse the security forces, our correspondent adds.
Police blocked the focal point of the demonstrations, Eskandari street, at both ends, sending bikers to disperse the protesters.
Both Mr Mousavi and Mr Karroubi have called for demonstrations in Iran in the light of the recent uprisings in Tunisia and in Egypt.
Earlier this month the two men, along with their wives, were detained in their respective homes in Tehran as protests were staged.
Thousands of their supporters took to the streets of Tehran on 14 February, amid clashes with security forces which left two dead.
On Monday one of Mr Karroubi's sons told the BBC Persian service he had been told his father had been "taken by security forces to an unspecified location".
The semi-official news agency Isna quoted state prosecutor Gholam Hossein Mohseni Ejehei on Monday as saying that Mr Mousavi and Mr Karroubi were not detained.
The two men ran as opposition candidates in the disputed June 2009 presidential election. Mr Mousavi said he was the actual winner and President Mahmoud Ahmadinejad was only re-elected through a rigged vote.
Hundreds of thousands of opposition supporters then took part in marches that were violently broken up by the security forces, including the Basij militia on motorcycles.