Some reports say shots have been fired in the air to disperse demonstrators.
The opposition had been planning to use Shia religious festivals this weekend to show their continued defiance of President Ahmadinejad's government.
Tensions have risen since influential dissident cleric Grand Ayatollah Hoseyn Ali Montazeri died a week ago aged 87.
Clashes were reported after his funeral in Qom, and in other cities since then.
Police chief Esmail Ahmadi Moghaddam has been quoted as warning that police would crack down hard against any attempts to use Shia religious processions to stage fresh protests against Mahmoud Ahmadinejad's government.
Opponents insist there was massive rigging when Mr Ahmadinejad was re-elected in June – a charge he rejects.
'Aggressive'
BBC Tehran correspondent Jon Leyne says opposition sources are building a picture of increasingly widespread and violent clashes.
Unconfirmed reports on reformist websites said there were protests in several areas of the capital, including the poorer areas of south Tehran, and that government forces were using tear gas to try to disperse demonstrators.
A witness quoted by Reuters news agency said opposition supporters had gathered in groups along a stretch of a main city centre route several kilometres long, but that police were not letting them join each other.
AFP news agency said police had "sought to disperse about 200-300 people" planning to demonstrate in central Enghelab square as part of a Shia festival.
Government forces are said to reacting aggressively – beating protesters with batons and smashing the windscreens of cars.
Iran's former pro-reform President, Mohammad Khatami, is expected to speak at a Shia gathering in Tehran this evening.
Shia religious festivals on 26-27 December mark the death of a grandson of the Prophet Muhammad in the 7th Century.
The main date set by reformists for demonstrations is Sunday.
