Iranian police broke up a crowd of several hundred supporters of moderate presidential candidate Mirhossein Mousavi in a Tehran square early on Saturday, a Reuters witness said.
The incident took place after the election commission said President Mahmoud Ahmadinejad had a strong lead over Mousavi in early counting of votes from Friday's presidential election. Mousavi earlier said he had won the election.
Police intervened against the crowd of chanting Mousavi supporters in Tehran's downtown Fatimi square -- where his campaign headquarters are located -- after warning them to disperse, the witness said.
Police have banned street gatherings until official election results are announced but another Reuters witness said dozens of Ahmadinejad supporters were celebrating in a Tehran boulevard, waving flags and some of them holding candles.