Sporadic demonstrations in Tehran and other Iranian cities have been reported on the anniversary of the disputed presidential election.
A massive security presence prevented any larger gatherings to protest at Mahmoud Ahmadinejad's re-election.
Opposition leaders earlier called off protests, saying they did not want to cause the loss of innocent lives.
The BBC's Tehran correspondent says the day has left the opposition once again with no sense of direction.
And many Iranians are seething with frustration, Jon Leyne reports.
The government insists Mr Ahmadinejad was re-elected by a landslide last year and has dismissed protests as a Western plot.
Reports of the new unrest could not be verified independently because of Iranian foreign media restrictions.
According to eyewitnesses, and judging from footage posted on the internet, members of the security forces were patrolling in large numbers across Tehran.
The intelligence ministry sent out a text message warning people: "In case of any illegal action and contact with the foreign media, you will be charged as a criminal."
Later the text message system was turned off, as often happens at moments of tension, our correspondent says.
Despite these measures, there were some demonstrations, particularly outside Sharif University in Tehran, and at some major road junctions.
But the protests appear to have been broken up before they could gather momentum, our correspondent says.
Outside Iran, a small pro-opposition rally was held near the Iranian embassy in the Japanese capital Tokyo.
Opposition sources say their supporters went out on to 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.
They came on to the streets in their millions a year ago, our correspondent says.
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.
The opposition seem to have run out of ideas and many Iranians are now reduced to sullen acquiescence, our correspondent adds.
Meanwhile, government's next big problem looks to be the economy: with falling oil revenues, it could be, fairly rapidly, running out of money.