At least 20 people have been killed by security forces in Yemen, doctors say, continuing a bloody crackdown on protesters that started on Sunday.
Snipers in Sanaa fired from rooftops at a protester camp, killing bystanders including a child, witnesses said.
Government forces have also begun shelling areas held by soldiers loyal to the protesters.
The opposition has promised to carry on its campaign to oust President Ali Abdullah Saleh.
For months, thousands of people have been waging a campaign to depose Mr Saleh, who is currently in Saudi Arabia recovering from a bomb attack in June.
The opposition believes the government is deliberately orchestrating the violence to derail any chance of agreement.
But a Yemeni minister strongly denied reports that the authorities had attacked genuine demonstrators, telling the BBC government forces were being attacked by militants sympathetic to al-Qaeda.
The US and EU nations were among members of the UN Human Rights Council who used a meeting in Geneva on Monday to urge Yemen's government to stop using force against peaceful protesters.
Meanwhile, as the violence intensified, envoys from the UN and the Gulf Co-operation Council arrived in Yemen, in a new attempt to negotiate a handover of power from Mr Saleh. more