More than 50 people have been killed in demonstrations in the southern Yemeni city of Taiz since Sunday, the UN says.
Reports, which "remain to be fully verified", also suggested hundreds had been injured in the city, the UN human rights office said.
At least three more people died after Taiz security forces opened fire when rallies resumed on Tuesday.
Meanwhile, fresh fighting in Sanaa ended a truce between tribesmen and forces of President Ali Abdullah Saleh.
Mr Saleh, who has ruled since 1978, has refused to step down despite months of protests against his rule and concerns about the war slipping into civil war.
'Fiercest so far'
The UN said reports indicated that those in Taiz had been killed by "Yemeni army, Republican Guards and other government-affiliated elements who forcibly destroyed the protest camp in Horriya Square using water cannons, bulldozers and live ammunition".
Navi Pillay, the UN High Commissioner for Human Rights, has urged all parties to find a peaceful solution to the conflict, saying that the "bloodshed must stop".
"Such reprehensible acts of violence and indiscriminate attacks on unarmed civilians by armed security officers must stop immediately. Further violence will only yield more insecurity and move the country further away from a resolution to this political crisis," she said.
At least 100 people are also believed to have been arrested over the weekend, while dozens of others remain unaccounted for.
EU policy chief Catherine Ashton said she was "shocked" by the use of force and live ammunition, and described attacks on medical centres as "appalling".
In Sanaa, battles have again erupted in part of the capital that is home to Sadiq al-Ahmar, the leader of the powerful Hashed tribe, breaking a truce agreed on Sunday after five days of fighting. At least one person has been killed.
A resident, Talal Hazza, said an artillery shell exploded outside his home and another destroyed his neighbour's house.
The fighting went on for hours overnight, witnesses said.
"Last night's clashes were the fiercest so far; my children and I couldn't sleep all night because of the heavy shooting," Mohammed al-Quraiti, a resident, told Reuters.
In the coastal city of Zinjibar, Islamic militants killed at least five Yemeni soldiers, security officials said, as clashes continued on Tuesday.
Government forces have been fighting to re-claim control of the city after it said it had been seized by gunmen from al-Qaeda in the Arabian Peninsula (AQAP).
The country's opposition has accused President Saleh of allowing the city to fall to al-Qaeda to stoke fears of a possible Islamist takeover should he leave power, according to AFP news agency.
At least 320 people have been killed in various fighting in Yemen since protests against Mr Saleh to quit began in February, inspired by uprisings in countries such as Tunisia and Egypt.