Kaleme, a Web site organized by the opposition, said security forces were using tear gas on demonstrators at Tehran University, CNN reported. Other residents compared Tehran to an armed camp because of the number of government agents patrolling the capital.
The BBC said by evening members of the Basji, the semi-official militia, could be heard in victory chants.
The protests were called in response to rumors of the jailing of Mir Hossein Mousavi, who ran against President Ahmed Ahmadinejad in 2009, and Mehdi Karrubi, head of the National Trust Party. The Fars news agency, which is tied to the government, said both were under house arrest with the only limit on their activities a ban on having contact with "suspicious elements."
Karrubi's son told the BBC his father was being held in an "undisclosed location."
Both men called for anti-government protests before they disappeared from public view.
The government hailed the resignation of Egyptian President Hosni Mubarak after days of street protest. But it has refused to make any concessions to protesters in its own cities.