Government forces in Yemen have continued firing shells at a protester camp in the capital, witnesses say.
Explosions rocked Sanaa all night, and at least two people had died in the shelling, according to doctors.
Government forces launched a crackdown on the protesters on Sunday, killing more than 50 people in two days.
Groups of protesters have occupied various parts of the capital for most of the year, calling for the ousting of President Ali Abdullah Saleh.
Mr Saleh, who has ruled the country for more than three decades, has been in Saudi Arabia since June, when he was seriously injured in a rocket attack on his presidential compound.
The president has refused to stand down and is promising to return to the country.
Rocket attack
The latest violence is the worst the country has seen for several months.
Witnesses say government snipers have been shooting protesters from the rooftops in Sanaa - although ministers have consistently denied that their soldiers have targeted civilians.
Government forces have also been involved in full-scale combat with an army unit that defected to the protesters months ago.
Witnesses say military aircraft have been targeting positions held by the protest-supporting troops, as well as constant shelling.
One report said troops loyal to the protesters on Monday seized a base of the republican guards - an elite force run by the president's son Ahmed.
Analysts say the final battle for control of the country could pit the republican guards against the army units loyal to the protesters and their tribal allies.
The BBC's Middle East correspondent Jon Leyne says the civilian protesters are increasingly caught in the middle of a conventional military battle between the two sides.
After two days of bloodshed in the capital, a third day of violence began before dawn.
"We were walking back from prayers. All of a sudden a rocket hit close by from out of nowhere, and some people fell down," protester Manea al-Matari told Reuters news agency.
"And then a second one came and that's when we saw the two martyred," he said.
Dr Mohammed al-Qubati, who has been running a field hospital at the protest camp, told the agency that he had seen the two people killed in the attack.
Mass protests and killings by security forces have also been reported in the cities of Taiz and Aden in recent days.
The Yemeni government has strongly denied reports that the authorities had fired on peaceful demonstrators, telling the BBC the unrest was initiated by al-Qaeda-linked forces within the opposition.
Envoys from the UN and the Gulf Cooperation Council arrived in Yemen on Monday in a bid to sort out a deal to end the bloodshed.
