Forces loyal to the embattled Yemeni president killed 20 protesters as they dispersed a sit-in in Taez, an organiser said on Monday, as suspected Al-Qaeda gunmen killed six soldiers in the south.
Security service agents backed by army and Republican Guard troops stormed the protest against President Ali Abdullah Saleh in Freedom Square in the centre of Yemen's second-largest city during the night, shooting at the demonstrators and setting fire to their tents, protesters said.
"At least 20 protesters have been killed," one of the protest organisers said.
Troops backed by tanks stormed a field hospital and detained 37 of the wounded receiving treatment there, among hundreds of people rounded up as security forces pursued the protesters into nearby streets, medics and organisers said.
"This was a massacre. The situation is miserable. They have dragged the wounded off to detention centres from the streets," activist Bushra al-Maqtari told AFP.
Protesters said that the square had been entirely cleared, while security forces stormed a nearby hotel and arrested several journalists.
The Common Forum opposition coalition condemned the "crimes against humanity" committed by Saleh's "remaining military and security forces and armed militias."
It warned the veteran president he would be "held personally responsible for his continued crimes against the people."
"These crimes do not get forgotten with time. They are being monitored and documented, and those who have committed them, and who provided arms and money, will not escape justice."
The four-month-old sit-in in Taez was the longest-running protest against Saleh's rule.
The clashes erupted on Sunday evening outside a nearby police station as some 3,000 people gathered to demand the release of a detained protester.
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Police fired warning shots then fired into the crowd when the demonstrators refused to leave, a local committee of the "Youth of the Revolution" group said.
On March 18, 52 people died when regime loyalists attempted to break up a similar protest against Saleh's rule in University Square in the capital Sanaa. The president declared a state of emergency after the bloodshed.
More than 200 demonstrators have been killed since the protests first erupted in Yemen. Scores more have died in armed clashes between loyalist troops and dissident tribesmen.
In the south, meanwhile, suspected Al-Qaeda militants killed six Yemeni soldiers, an army officer and a security official said.
The officer said that two soldiers from the 25th mechanised brigade, the headquarters of which is under siege in Zinjibar, were killed by a rocket-propelled grenade.
The security official said that militants ambushed a military convoy outside Zinjibar during the night, killing four soldiers. At least seven soldiers were wounded, a medic said.
A security official said on Sunday that suspected Al-Qaeda gunmen had taken control of most of Zinjibar, the capital of Abyan province, in three days of fighting during which officials and medics said at least 21 people were killed.
Witnesses said that aircraft were carrying out strikes on suspected Al-Qaeda positions to the east of the city on Monday, while there were also unconfirmed reports of naval shelling in the area of Zinjibar, which sits a few kilometres (miles) from the coast.
Four suspected Al-Qaeda fighters were killed in overnight fighting in Zinjibar, another security source said, but a source close to the gunmen who control much of the city said that only two were killed.
Dissident army commanders accused Saleh of surrendering the province to "terrorists."
The Common Forum charged that he had "delivered Zinjibar to groups that he has formed and armed, to continue to utilise the spectre of Al-Qaeda to frighten regional and international parties."
The opposition grouping signed up to a deal brokered by impoverished Yemen's wealthy Gulf neighbours under which Saleh, in power since 1978, would hand power to the vice president within 30 days in return for a promise of immunity from prosecution.
The president initially agreed to the plan but then repeatedly set new conditions for signing it, prompting the six-nation Gulf Cooperation Council to suspend its mediation efforts.