Fifteen people were injured four of them critically, when a car was driven into a protest march in the Yemeni capital calling for President Ali Abdullah Saleh to be put on trial.
A Reuters witness saw the unmarked car careen into the crowd of tens of thousands who had marched past the town hall without obstruction from police there.
Protesters said they believed the car had been driven by a Saleh supporter, whom they detained after attacking the vehicle and setting it on fire, Reuters reports.
Nine months of anti-government protests have deeply divided impoverished Yemen, pushing the Arabian Peninsula country to the verge of a civil war and humanitarian crisis.
In the capital Sanaa, supporters and opponents of Saleh are increasingly entrenched, with government forces and dissident army units controlling separate parts of the city.
The European Union envoy to Yemen was quoted Monday as saying Saleh had moved a step closer to handing power to his deputy by accepting a U.N. formula to ease a transition.
Saleh has repeatedly said he is ready to leave office, only to back out at the last minute.