Five protesters including a schoolboy were killed in fresh bloodshed in Yemen on Saturday as clashes between police and anti-regime demonstrations raged across the country.
Two doctors at the scene in Sanaa said that toxic gas, rather than ordinary tear gas had been used against the protesters, a claim dismissed as slander by the authorities.
Britain, the European Union and the United Nations condemned the crackdown.
Security forces in the impoverished country, a key US ally in the war against Al-Qaeda, fired lived rounds and tear gas at demonstrators camping at University Square since February 21, killing one, protest organisers said.
More than 30 protesters were wounded by the gunfire, while and hundreds more suffered the effects of the tear gas, some passing out and others going into spasms, medics said.
A sniper shot dead another man as he walked with a group of demonstrators to the square, an opposition party member said.
Police shot dead the schoolboy in the southeastern city of Mukalla as they tried to disperse a student demonstration, witnesses and medics said.
And two other protesters were later killed in the southern port city of Aden: one by police when they opened fire to disperse a demonstration and the other when demonstrators set fire to a police station in the city.
A medical official said hundreds of angry people had set ablaze the police station to protest the death of the protester earlier in the day. Several people were also wounded by gunfire, he said
The violence comes a day after 14 protesters were wounded in protests across the country, which is already battling secessionist unrest, a Shiite sectarian rebellion and jihadists from Al-Qaeda's Arabian Peninsula offshoot.
The dawn assault at Sanaa's University Square targeted demonstrators who had breached a concrete police barrier where activists have been staging a sit-in for almost three weeks, calling on President Ali Abdullah Saleh to step go.
Parts of Sanaa resembled a battleground as people passed out in the street and convulsed after inhaling gas fired at the demonstrators.
"This isn't tear gas," said Iraqi doctor Hussein al-Joshaai, a nerve specialist who was at the scene.
"This is poison gas that disables the nervous and respiratory systems. People hit by this gas pass out," he added.
Another doctor, Abdulwahab al-Inssi, said: "Those wounded today couldn't have been hit by tear gas grenades. They are suffering spasms."
The interior ministry denied the allegations as "baseless slander."
It accused protesters of opening fire at security forces who had tried to prevent clashes between demonstrators and residents near the square. It said 161 police were injured.
Street battles raged all morning as security forces blocked roads to the square and prevented ambulances from evacuating casualties, protest organisers said.
A security official said police had only planned to "return the demonstration to its size of yesterday because the expansion of the sit-in has disturbed residents."
European Union foreign affairs chief Catherine Ashton, condemning the government crackdown, said: "I sincerely regret the loss of life and that many were injured."
She urged Saleh's government to honour promises he had made two days ago to protect demonstrators and uphold their right to free assembly.
United Nations Secretary General Ban Ki-moon is "greatly concerned" about violence in Yemen and condemned the "excessive use of force" against "peaceful demonstrators," his spokesman said.
And British Foreign Secretary Hague said: "I was shocked by the unacceptable violence seen in Sanaa today."
US ambassador Gerald M. Feierstein described Saturday's clashes as "dangerous" and called for "dialogue and negotiation."
Saleh has insisted he will see out his term until 2013 while offering to devolve power to parliament after a referendum on a new constitution this year, an offer that the United States has welcomed.
But opposition groups had already dismissed the promise of constitutional change and have vowed to escalate protests until Saleh, in power since 1978, resigns.
More than 30 people have been killed since the unrest erupted in late January.
US special forces troops are in Yemen helping to train anti-terror forces as the country struggles to contain Al-Qaeda's local offshoot -- described by a State Department official as the biggest threat to the US homeland.