At least 25 people were shot dead and hundreds wounded when security forces opened fire on protesters in the Yemeni capital of Sanaa.
The violence began when demonstrators camped in a square in Sanaa tried to charge police lines in a dramatic escalation of protests against President Ali Abdullah Saleh.
A Reuters witness saw security forces firing at protesters from buildings overhead, as well as using water cannon and tear gas.
Thousands have camped out in ramshackle tents for eight months to demand an end to Mr Saleh's 33-year rule.
Medics estimated that some 342 suffered gunshot wounds, with 36 in critical condition.
At least one of the dead had been hit by a rocket-propelled grenade, they said.
Frustrated by their failure to oust Mr Saleh, who is recovering in neighbouring Saudi Arabia from a June assassination attempt, protesters were keen to step up demonstrations.
