At least 150 people were wounded as Yemeni security forces tried to break up a rally in the Red Sea city of Hudaida demanding an end to President Ali Abdullah Saleh's rule, a doctor and witnesses said on Wednesday.
The impoverished Arabian Peninsula state, neighbour to oil giant Saudi Arabia, has been hit by weeks of protests trying to shake loose Saleh's 32-year grip on power.
Both pro and anti-government factions appear to have increasingly resorted to violence in the struggle.
A doctor treating protesters at a sit-in in Hudaida said hundreds of security forces and plainclothes police, all armed, attacked the demonstrators.
"They attacked the protesters and wounded around 120 people," he told Reuters by telephone. "They were using tear gas, rubber bullets, live fire and bats."
Demonstrators contacted by Reuters said they were calling on private hospitals to send ambulances and asked Yemenis to donate blood to help treat the wounded.
One protester said mostly plainclothes police had attacked the sit-in and were still clashing with protesters.
"Special forces, central security forces and police, most of them in civilian clothing, are surrounding the protesters," said Mohammed Muajem. "The main hospital is now at full capacity."
Two protesters told Reuters some of the wounded demonstrators were chased down by security forces and then beaten in hospital, but this could not be immediately verified.
The United States, which has long seen Saleh as a bulwark against an aggressive and agile al Qaeda wing based in Yemen, has condemned the bloodshed and backed the right to peaceful protest. But it has also insisted only dialogue can end the political crisis.
In a further escalation on Wednesday, a leading activist in Taiz said plainclothes police targeted and beat women at a rally in the city, 200 km (125 miles) south of capital Sanaa. At least eleven were wounded, Bushra Al-Maqtari said.