Leading reformist politicians signaled their intent to continue protest over what they say was a fraudulent election.
Thousands of people clashed with security forces in the streets after a symbolic 40-day period since the death of protesters. Thirty years ago, such protests helped bring down the Shah.
President Mahmoud Ahmadinejad fired his intelligence minister Sunday after being forced to remove his first vice president, a close adviser.
Street clashes between security forces and supporters of challenger Mousavi picked up after dark on Sunday after President Ahmadinejad addressed thousands of his supporters in Tehran.