Yemen's powerful Republican Guard forces are firing rifles and using tear gas and water cannons to attack a march by 100,000 protesters demanding the president be put on trial.
The forces are commanded by President Ali Abdullah Saleh's son and attacked the protesters as they converged on the capital after a four-day march from a southern city.
One of the organisers, Abdel Nasser al-Kamali, says 30 people were injured in Saturday's attack. He says tens of thousands of other regime opponents from inside the capital joined the demonstration.
Participants in what has been dubbed the March of Life followed a 170-mile (270-kilometer) route from the city of Taiz, a centre of the opposition.
They object to a deal under which Saleh would step down in return for immunity from prosecution.