Dozens were injured when police and loyalists of the ruling General People`s Congress party attacked protesters occupying Sanaa`s University Square with live gunfire and tear gas, witnesses said.
Six demonstrators were shot in the head during clashes with police overnight in the southern city of Aden, with two dying of their wounds, medical officials said.
Witnesses said police sharpshooters fired on demonstrators from rooftops in the city as protesters attacked and burned a police station and chanted slogans calling for the fall of the regime.
A 25-year-old protester was also killed Sunday evening as security forces opened fire on people who had set fire to another police station in Aden and looted its weapons, including assault rifles, a hospital source said.
The deaths brought the toll since Saturday to eight as pro-democracy opposition groups and students escalated their campaign to oust autocratic President Ali Abdullah Saleh after 32 years in power.
The United States joined Britain, the European Union and the United Nations in condemning the violence, in a country where US special forces are helping train local units engaged against Al-Qaeda`s offshoot in the Arabian Peninsula.
"People everywhere share the same universal rights to demonstrate peacefully and to freely assemble and express themselves. Violence must cease immediately," State Department spokesman Philip Crowley said.
"We call on the Yemeni government to quickly investigate these incidents and take all necessary steps to protect the rights of all its citizens, in accordance with President Saleh`s commitments."
United Nations chief Ban Ki-moon condemned the "excessive use of force" against peaceful demonstrators and urged all sides to engage in dialogue, his spokesman said.
British Foreign Secretary William Hague said late Saturday the violence against demonstrators was "unacceptable" and called on all British citizens to leave the country.
"This is in direct contradiction to the president`s recent announcement on constitutional reform and fresh elections, which we have welcomed," Hague said in a statement.
In a speech to tens of thousands of people at Sanaa`s stadium Thursday, Saleh promised to protect protesters from violence and offered to hold a referendum on a new constitution which would devolve power to parliament.
The United States, which sees Saleh as a pillar of stability in the deeply tribal nation, has welcomed the gesture but Yemen`s parliamentary opposition says the president has lost all credibility and must resign this year.
In the capital Sanaa, thousands more demonstrators demanding democratic reform, jobs and an end to corruption poured into University Square despite fierce clashes there on Saturday in which one person was killed.
The square has become a cauldron of dissent since and a pro-democracy tent city was established there on February 21, amid a wave of unrest that has swept the Arab world.
Streets to the square were blocked with tents earlier Sunday, many flying flags and other symbols denoting the provinces of demonstrators who have come from around the country to join the opposition movement.
More than 30 protesters were wounded by the gunfire and tear gas on Saturday morning as police tried to push the campers back into the heart of the square.
On Friday, 14 protesters were wounded in demonstrations across the country, which is already battling secessionist unrest and a Shiite sectarian rebellion. (*)