Demonstrations were held on Thursday to denounce the deaths of anti-government protesters [Reuters]

Tens of thousands of people have rallied across Yemen, denouncing the deaths of at least 13 anti-government protesters killed in the capital Sanaa.

Demonstrators gathered in heavy rains on Thursday to condemn the crackdown the previous day in several central cities, as well as in Taiz and in Saada.

Medics were treating the injured at hospitals across the capital.

"Most of the injuries were lethal, most of them were with live ammunitions and knives and stones; 780 cases arrived, 125 were with live bullets.Thirteen were martyrs [died]," a doctor at a Sanaa hospital told the Associated Press news agency.

An injured man at the same hospital said Saleh was clamping down on the Yemeni youth. "The chair [presidency] is not more precious than the Yemenis. Leave if you are a man with pride," he said.

The opposition bloc, which has warned government-orchestrated violence against protesters could derail a draft deal to end the political standoff, agreed on Tuesday to take part in a transitional government under the Gulf-negotiated pact, which gave Saleh immunity and his family.

The balance of power has tipped against Saleh, who has ruled for 32 years, after weeks of violence, military defections and political reversals.

Seeing political allies desert him en masse, the Yemeni leader agreed in principle to a proposal by Gulf Co-operation Council (GCC) foreign ministers to step down in exchange for immunity from prosecution for himself and top aides.

But analysts say that allowing Saleh, who has been a key ally of the West against al-Qaeda, to stay on for another month could leave a room for further trouble in the poorest Arab state long on the brink of collapse.

UN 'concern'

In a statement issued on Thursday, Ban Ki-moon, the UN secretary-general, expressed "concern" at the latest violence.

Ban said he welcomed "the efforts being made to promote a peaceful transition that will be acceptable to all," but he appealed "to all concerned in Yemen to exercise utmost restraint and desist from provocative acts."

The US embassy in Sanaa also urged government security forces to refrain from using violence against demonstrators.

Yemen's unrest started in early February, inspired by similar revolts in the Arab world. Opposition parties continue to accuse Saleh and his relatives of  "massacres" and a crimes against humanity.

Source: Agencies