Police moved in on protesters who had been camped out in University Square since February 21, they said.

One protester was killed and nearly 300 were wounded or affected by the tear gas, said a medical committee set up by the protesters.

An AFP correspondent said the security forces attacked demonstrators who had stretched the sit-in to neighbouring streets beyond concrete blocks installed by police to demarcate the limit of the protest area.

"By 3:00 am, security forces began to harass protesters, and started to dismantle tents and megaphones," protester Mohammed Saeed, 20, told AFP.

He said the forces had brought in reinforcements after protesters confronted them, and that by 5:30 am (0230 GMT) police fired live rounds and tear gas grenades.

A volunteering doctor at the square charged that the security forces used a poison gas whose type was difficult to determine, while treatment to deal with its effect was not available at the field clinic.

"These are not tear gases. These are poisonous gases that disable the nerve and respiratory systems. People hit by those gases pass out," said Iraqi doctor Hussein al-Joshaai, a nerve specialist.

"We do not have a treatment for this. We have asked the authorities to send us medicines but we are still waiting," he added.

The brutal crackdown came despite Yemeni President Ali Abdallah Saleh's promise in a speech Thursday that he would protect demonstrators, whether they were for or against his rule.

Saleh also pledged to devolve power to parliament through constitutional reforms that would be voted on in a referendum later this year.

His offer appeared to bow to pressure after more than a month of violent protests, was swiftly rejected by the opposition, which is demanding his immediate resignation.

The United States was quick in hailing the offer by Saleh, a key ally of Washington in its war against Al-Qaeda which has been active in the south Arabian Peninsula country.

US President Barack Obama's top anti-terror adviser John Brennan on Friday called Saleh to welcome his pledge and urged the opposition to support the plan.

"Brennan reiterated that representatives of all sectors of the Yemeni opposition should respond constructively to President Saleh's call to engage in a serious dialogue to end the current impasse," the White House said in a statement.

Protesters have been calling on Saleh, in power since 1978, to step down despite his pledges of reform.

On Friday, 14 protesters were wounded, including three who appeared to have been shot with live bullets, when police attacked demonstrators in the southern city of Aden, on a day of anti-regime protests across the impoverished country.