Demonstrators wearing the trademark green colour of Mir Hossein Mousavi chanted
slogans condemning the results that gave 62.6 per cent of the vote to President Mahmoud Ahmadinejad.
Protesters set fire to tyres 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.
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% 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% participation and warned
opposition candidates to "avoid provocative behaviour".
"I assume that enemies intend to eliminate the sweetness of the election with
their hostile provocation," he said in his televised address.
He called the results a "divine assessment" and called on all the candidates
to support the president.