A statement from Mousavi posted on his Web site condemned what he described as the "manipulation" of election results.
Demonstrators wearing the trademark green color of Mir Hossein Mousavi chanted slogans condemning the results that gave 62.6 percent of the vote to President Mahmoud Ahmadinejad. Protesters set fire to tires outside the Interior Ministry in the most serious unrest in Tehran in a decade.
Witnesses also said a commercial bank elsewhere in the city was set on fire.
Police attacked the demonstrators near the Interior Ministry, where the election results were announced, beating them with clubs and smashing cars. Police also moved to disperse any large gatherings of people around the city.
An Associated Press photographer saw a plainclothes security official beating a woman with his truncheon.
In another main street of Tehran some 300 young people blocked the avenue by forming a human chain chanted "Ahmadi, shame on you. Leave the government alone."
Iranian authorities said that Mousavi only took 33.75 percent of the vote in a contest that was widely perceived to be much closer than the official results.
Iran's supreme leader, Ayatollah Ali Khamenei, who has final say on all state matters, thanked the people for their record 85 percent participation and warned opposition candidates to "avoid provocative behavior."
