Iran’s state TV is showing thousands of pro-government demonstrators rallying to mark the anniversary of the 1979 Islamic Revolution.
The opposition has vowed to stage counter demonstrations, but eye-witnesses say police have deployed hundreds of forces in central Tehran to confront them.
Opposition members went on roof tops late Wednesday and shouted “God is greatest” in protest - echoing similar cries after the disputed June election as well as anti-shah protests more than three decades ago.
Since the election that brought President Mahmoud Ahmadinejad to power, opposition protesters have repeatedly taken to the streets.
Thursday’s pro-government rallies in Tehran will end at Azadi square, where the president will address the crowd.