At least seven protesters were killed and dozens wounded on Tuesday as gunmen loyal to President Ali Abdullah Saleh opened fire on demonstrators in the Yemeni capital, medics and the opposition said.
It was the third time in four days that demonstrators had attempted to march from their base in Sanaa's Change Square on loyalist-held areas of the capital to be met by deadly gunfire.
Leading activist Sakhr al-Wajih told a press conference that six protesters were killed and 59 wounded.
A medic said a seventh slain protester was received at the state-run Republican hospital, along with four wounded, one in a serious condition.
Doctors at the makeshift clinic that protesters have set up in Change Square, where they have camped out for months, said dozens of the injured had sustained bullet wounds, while others were being treated for tear gas inhalation.
Demonstrators came under fire from gunmen and police in Ziraa Street as tens of thousands marched from Change Square to the loyalist-held Al-Qaa neighbourhood, witnesses said.
Armed loyalists had erected tents in the street to block the march. The protesters came under attack after passing the tents towards Al-Qaa, a district where several government buildings are located.
Witnesses said thousands of demonstrators were briefly trapped in Al-Olfi Square on Ziraa Street, coming under fire from hundreds of loyalist security forces and gunmen.
A second march from Change Square through Tunis Street passed off peacefully, after being diverted by loyalists troops without any clashes, witnesses said.
In a letter to UN chief Ban Ki-moon, Yemen's new Nobel Peace Prize winner Tawakkul Karman called on the world body to take "immediate and decisive action to stop the massacres and hold the perpetrators accountable."
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The UN human rights office condemned the killing of peaceful protesters by security forces.
"We condemn in the strongest terms the reported killing of a number of largely peaceful protesters in Sanaa and Taez as a result of the indiscriminate use of force by Yemeni security forces since Saturday," said Rupert Colville, spokesman for the UN High Commissioner for Human Rights.
"We are extremely concerned that security forces continue to use excessive force in a climate of complete impunity for crimes resulting in heavy loss of life and injury, despite repeated pledges by the government to the contrary," he added.
Colville said an independent and transparent international inquiry should be held into the killings so that those responsible could be prosecuted.
Organisers had vowed to press ahead with the two marches despite the deadly response from security forces on Saturday and Sunday to attempts by the protesters to leave parts of the city under the control of the dissident First Armoured Division, which rallied to the opposition in March.
Saleh's forces shot dead 12 protesters on Saturday, while four demonstrators and two soldiers of the armoured division were killed on Sunday.
Another eight people were killed and 27 wounded in street battles in the capital between Saleh loyalists and opponents on Sunday night, medics and a tribal source said.
A family of five were killed on Monday night when a rocket hit their home in north Sanaa.
In Yemen's second-largest city Taez, one protester was killed and seven wounded when Saleh loyalists opened fire on a demonstration late on Monday, witnesses and medics said.
Despite mounting pressure from Western governments and the six-nation Gulf Cooperation Council, Saleh has for months refused to sign a deal brokered by the GCC for him to hand over power in return for immunity from prosecution.
According to a letter from Yemen's youth movement sent to the United Nations earlier this month, at least 861 people have been killed and 25,000 wounded since protests first erupted in January against Saleh's 33-year rule.