Iranian riot police armed with clubs and riding motorbikes fired tear gas on Monday as about 1,000 opposition demonstrators gathered in Tehran in defiance of an official ban, witnesses said.
One witness said police in helmets and wielding steel clubs and cables fired at least seven rounds of tear gas to disperse one group of about 200 protesters as they started to chant Allahu Akbar (God is Greatest).
The police, many wearing black bullet-proof vests and riding motorbikes, were also armed with guns used to fire rubber bullets, witnesses said, while helmeted members of the Islamic volunteer Basij militia backed them up carrying clubs.
Police arrested 50-60 protesters, witnesses said.
The demonstration -- which included many women -- broke up nearly two hours later as protesters were confronted by more than 500 riot policemen and Basiji militiamen, witnesses said.
A helicopter hovered overhead as the protesters gathered in Tehran's Haft-e Tir square -- a popular Tehran shopping district -- on the latest day of demonstrations over the disputed presidential election.
"There are about 1,000 people gathering around the square," one witness told AFP. "Riot police are taking position to charge."
A call to rally at Haft-e Tir was issued on some social networking sites, including Twitter, to pay tribute to a woman known by her first name Neda, allegedly killed by gunfire on Saturday.
A video, viewed by hundreds of thousands around the globe, showed a bloodstained young woman said to be Neda after she was reportedly killed by a bullet in Tehran during confrontations between protesters and police.
The foreign media is banned from covering demonstrations, effectively keeping their journalists off the streets, but Iranians have been using sites such as Twitter and YouTube to get news and video footage to the outside world.
The Revolutionary Guards, an elite force set up to protect the Islamic republic in the wake of the 1979 revolution, has vowed to crush protests over the June 12 presidential election.
The opposition led by defeated candidate Mir Hussein Mousavi claims the poll that returned hardline President Mahmoud Ahmadinejad for a second four-year term was rife with irregularities.