At least eight people have been injured in an apparent grenade attack as anti-government demonstrators clashed with supporters of Yemen's long-time ruler and riot police.
Security forces fired tear gas and shots in the air to disperse the crowd on what organisers called a "Friday of Rage".
The grenade attack came in the city of Taiz, seriously wounding at least eight people in the blast and stampede that followed, witnesses said.
Riots also flared in the southern port of Aden with police shooting dead one demonstrator after cars and a local government building were set ablaze, officials said. It was the eighth straight day of protests in Yemen inspired by uprisings in Egypt and Tunisia.
Demonstrators in the Arab world's poorest country are calling for the removal of President Ali Abdullah Saleh - a key US ally in fighting al Qaida terrorists - who has ruled the country for 32 years.
Mr Saleh is already facing threats from al Qaida militants saying they want to oust him, a southern secessionist movement and an on-and-off armed rebellion in the north. To try to quell the new outbursts of dissent, Mr Saleh pledged to meet some of the protesters' demands and has reached out to tribal chiefs.
But a major chief from Mr Saleh's own tribe was critical of his policies and threatened to join the protesters - an apparent attempt to put pressure on the embattled leader.
For now, most of the protesters are made up of students, educated professionals and activists who used social media sites Facebook and Twitter in summoning people to the streets for the "Friday of Rage" following noon prayers.
Tens of thousands responded in the capital of Sanaa, the southern port of Aden and the political hotbed of Taiz. Some websites also referred to the day as "Friday of the Beginning."
The crowd in Sanaa, chanting anti-government slogans, was met by a heavy deployment of riot police and hundreds of Saleh supporters. The pro-and anti-government sides attacked each other with rocks, and the riot police began firing in the air and launching tear gas canisters. At least four people were hurt seriously enough to be taken away by ambulances.