Tens of thousands of Yemenis in the capital chanted "No deal, no maneuvering, the president should leave".
Hundreds of thousands of Yemenis have gathered in cities and towns across the country to protest what they consider President Ali Abdullah Saleh's latest attempt to avoid stepping down.
"No deal, no maneuvering, the president should leave," protesters in the capital Sanaa shouted on Tuesday, demonstrating against a decision made by Saleh a day earlierto authorise his vice president to negotiate with the opposition and sign a transition plan on his behalf.
The opposition insists that the president sign the power transfer plan himself.
The impoverished country has seen many acts of violence since nationwide pro-democracy protests broke out in February, calling for an end to Saleh's 33-year rule.
"All sides may be guilty of using and abusing peaceful protesters and the civilian population in this increasingly violent power struggle," the 23-page UN report concluded.
The UN cited allegations that the government had disrupted telecommunications, power and fuel supplies as a "form of collective punishment", while government officials blamed the opposition for having sabotaged an oil pipeline and power line.
Security forces had prevented wounded demonstrators from reaching hospital and in some cases have fired on ambulances, the report said.
Children have been subjected to killings, injury, suffocation from gas used on demonstrators, torture, arbitrary detention and recruitment by security forces, it said.
Navi Pillay, the UN high commissioner for human rights, called on the Yemeni government to "take immediate action to end attacks against civilians and civilian targets by security forces".
Deaths were reported in the southern Yemeni city of Zanjibar on Tuesday, where suspected al-Qaeda fighters killed three Yemeni soldiers and wounded five, medical and military officials said.