There is a huge security presence in Tehran and other Iranian cities, as the country marks the first anniversary of President Mahmoud Ahmadinejad's disputed re-election, witnesses say.
Opposition leaders have called off planned protests, saying they want to avoid the loss of innocent lives.
The government says Mr Ahmadinejad was re-elected by a landslide. It says the protests were just a Western plot.
So far there are no reports of any protests.
But opposition sources say their supporters did go out onto their roofs after dark on Friday night to chant Allahu Akbar (God is great), a gesture of defiance they began following Mr Ahmadinejad's re-election.
Sullen acquiescence
They came onto the streets in their millions a year ago. It was a spontaneous outburst of anger from huge numbers of Iranians who felt Mr Ahmadinejad had stolen the presidential election.
Since then the opposition have been steadily battered into submission, beaten up when they demonstrate on the streets, arrested and, they say, abused in prison.
Mr Ahmadinejad's government continues to be tellingly nervous about its hold on power. It has been steadily tightening its grip on the media and the internet, and even warning foreign exiles not to speak out.
The opposition seem to have run out of ideas. Many Iranians are now reduced to sullen acquiescence. The government's next big problem looks to be the economy: with falling oil revenues, it could be, fairly rapidly, running out of money.
