Iran's opposition forces called off plans to stage new protests against president Mahmoud Ahmadinejad, citing fears for their safety, but they have vowed to continue agitating against his re-election.
Police and the pro-government Basij militia were sent in their thousands to central squares in Tehran to put down any opposition unrest.
Government authorities last year used brute force to quell massive demonstrations that erupted after the president was returned to power.
There have been no such protests for months, but opposition leaders accuse Iran's rulers of crushing dissent by jailing opposition supporters and banning much media coverage.
By Middle East correspondent Anne Barker
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