Iranian opposition demonstrators clashed with security forces across the country during celebrations to mark the anniversary of Iran's 1979 Islamic Revolution.

Iranian opposition protesters chanted anti-government slogans in a Tehran neighborhood as government supporters held a massive rally to celebrate the 31st anniversary of Iran's Revolution. Eyewitnesses say demonstrations took place in dozens of Tehran neighborhoods, with security forces firing teargas at large crowds in major city squares.

Opposition leaders Mir Hossein Mousavi and Mehdi Karroubi had asked supporters both verbally and on their Web sites to turn out for a "peaceful demonstration" to mark the anniversary of the revolution. Karroubi's son reported that his father was attacked and injured by pro-government agents, forcing him to turn back. He added that his brother was arrested.

Iranian state TV showed what appeared to be a vast crowd gathered for the official ceremony at Tehran's Azadi or "Freedom" Square to hear President Mahmoud Ahmadinejad and other speakers hail the revolution and denounce Iran's enemies, including the United States and Britain.

At another Tehran square a group of angry opposition protesters began chanting anti-government slogans during an official rally, drowning out the cleric who was speaking and forcing him to stop.

Eyewitnesses say that protests in many parts of Tehran turned violent after security forces began firing teargas at crowds and chasing down protesters with motorcycles.

Clashes between security forces and opposition demonstrators also took place in a number of provincial cities. In Shiraz, a young man told Radio Farda that violent clashes with security forces took place as students turned out in the main square to demonstrate.

Opposition Web sites are reporting that hundreds of thousands of opposition protesters turned out in cities across the country, describing scores of violent clashes with security forces, VOA News reported.