Yemeni police firing live rounds and tear gas killed one person and wounded many more Saturday in a attack on anti-regime protesters camping at a main square in the capital.

The dawn assault forced back demonstrators who had breached the limits of a concrete barrier put up by police at Sanaa's University Square, where protesters have been staging a sit-in for almost three weeks.

It came two days after embattled President Ali Abdullah Saleh offered sweeping political reforms and pledged to protect protesters demanding an end to his three decades of iron-fisted rule.

At least one demonstrator was killed and 300 others suffered injuries, including from live bullets and tear gas, said a medical committee set up by the protesters.

A volunteer doctor accused the security forces of using poison gas, without being able to specify what type, as television footage showed many people passed out in the street.

The attack raged in the morning as security forces blocked all roads leading to the square, and prevented ambulances from reaching the area to evacuate the casualties, protest organisers said.

More than 30 protesters were shot with live rounds, while hundreds more were suffering from inhaling gases, medics at the square said.

A security official said meanwhile that police were not planning to storm the rest of the sit-in at the square, but that they were aiming to contain the demonstration.

"We are not thinking of storming the square. But we have to return the demonstration to its size of yesterday, because the expansion of the sit-in has disturbed residents," the official said, asking not to be named.

An AFP correspondent said the security forces attacked demonstrators who had stretched the sit-in to neighbouring streets beyond concrete blocks to demarcate 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 volunteer doctor at the square charged 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.

Meanwhile, police opened fire at student demonstrators in Taez, some 160 kilometres (100 miles) south of Sanaa, according to Bushra al-Maqtari, an organiser of anti-government protests in the city.

She said two people were wounded in the protest that answered a call for civil disobedience at schools.

The brutal crackdown came despite President 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.

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.