Supporters of Iran's defeated presidential candidate Mir Hossein Mousavi have held another big rally in northern Tehran, according to witnesses.
The protest came hours after Mr Mousavi urged them not to march in the centre of the city where President Mahmoud Ahmadinejad's supporters were rallying, the BBC reports.
Tough new restrictions on the foreign media mean the BBC is unable to confirm the scale of the opposition protest. It came despite an offer to recount votes the opposition disputes.
The media restrictions have been imposed amid apparent surprise and concern among the authorities at the scale of popular defiance over Friday's official election results, correspondents say.
Eyewitnesses, including a reporter for the state run Press TV station, say a huge opposition rally has gathered in north Tehran. Another rally of government supporters was held in central Tehran.
Mr Ahmadinejad was declared the easy victor of the presidential poll on Saturday, with results giving him 63% of votes against 34% for Mr Mousavi. But the opposition alleged widespread irregularities.
Iran's Guardian Council has ordered a recount, backed by Supreme Leader Ayatollah Ali Khamenei, but it will be limited to voting sites where candidates claim irregularities took place.
Mr Mousavi is calling for the entire election to be re-run. His supporters say a recount would be meaningless because they suspect millions of ballots have disappeared.
Anger at the official result saw hundreds of thousands of Mr Mousavi's supporters take to the streets on Monday - in a rally the size of which correspondents said not been seen in Tehran since the 1979 revolution.
A witness told the BBC that Tuesday's rally was even bigger than Monday's - though this cannot be independently confirmed.
Thousands of supporters of President Ahmadinejad staged their own rally in Vali Asr Square in central Tehran - some bussed in from the provinces, correspondents say.
The latest opposition rally comes despite a Mousavi spokesman urging supporters not to take part in another demonstration on Tuesday, amid fears of new violence.
United States President Barack Obama says he believes the voice of the people of Iran should be heard, not suppressed.
The State Department says the government has asked the internet messaging service Twitter to delay planned maintenance work to allow Iranians to communicate with each other and people outside Iran.
The planned upgrade work would have cut daytime services for Iranians. It was postponed to coincide with night-time in Iran.
A US official said the department had highlighted to Twitter that their site was an important horizontal form of communication for Iranians.
Authorities in Iran have shut down websites, including Facebook, and clamped down on text messaging in an attempt to bring the protests under control.