SANAA (Reuters) - At least 56 people were killed over two days in the deadliest crackdown yet on pro-democracy protesters in Sanaa, triggering fierce gun battles on Monday between soldiers who had defected to the opposition and those loyal to President Ali Abdullah Saleh.
Opposition forces said they agreed a truce with the government, but several rounds of gunfire and explosions were heard in the Yemeni capital and a government official said the two sides were still working on a cease-fire deal.
The military confrontation between opposition forces loyal to defected General Ali Mohsen and government troops was triggered by the government crackdown on protests, threatening a new and even more violent phase in Yemen's eight-month standoff.
Demonstrators demanding an end to Saleh's 33-year rule ratcheted up their protests on Sunday to try to break a stalemate, and government forces responded with heavy fire, while snipers shot at protesters from rooftops.
At least 30 people were killed on Monday, raising the death toll to 56 over two days. Among the dead was a journalist working for the Saudi channel al-Ikhbariya. A Reuters witness said he was shot in the face. The government reported two deaths among its soldiers on Monday.
Witnesses said government forces had traded heavy rifle and missile fire with troops loyal to Mohsen, who defected following an earlier crackdown in March which killed 52 people.
An escalation into outright military confrontation in Sanaa has been a major concern for many in Yemen, who fear this will make it even harder to reach a political settlement under which Saleh would step down, in theory paving the way for elections.
"Help me, oh my God look at this slaughter!" said a man carrying the bloodied body of his small child, killed by gunfire. "We were just in the car ... I stepped out to get some food and left my two boys in the car. I heard the older one scream. My little one was shot straight through the head."
"This is only going to get worse," one man shouted as he fled a new protest camp, staked out by protesters on Sunday night and attacked by snipers on Monday. Troops loyal to Mohsen jumped in trucks and sped towards the heavy cracks of gunfire.
"We will come back to protest later. I am afraid, but this is worth dying for," the man said. He was among hundreds rushing back to the relative safety of protesters' original sit-in area, dubbed Change Square, where they have camped out for 8 months.
Diplomats and Yemeni politicians scrambled on Monday to speed up a long-stalled transition plan under which Saleh, who is recovering in neighbouring Saudi Arabia from a June assassination attempt, would step down.
A source in Yemen's political opposition said they were meeting with government officials and diplomats to try and push through a deal. U.N. mediator Jamal bin Omar and Gulf Cooperation Council Secretary General Abdbullatif al-Zayani arrived in Sanaa on Monday and were expected to join the talks.
Zayani was expected to push for the signing of a Gulf-brokered transition plan which Saleh backed out of three times before.
"There's a possibility of trying to push through the Gulf plan for signing this week," an opposition source said.
But protesters vowed to march again on Tuesday to condemn the crackdown and lack of international response.
Several countries including the United States condemned the violence but gave little indication of how they planned to exert pressure on Saleh.
"The United States regrets the deaths and injuries of many people during protest marches in Sanaa yesterday. In this tense situation, we call upon all parties to exercise restraint," the U.S. embassy in Sanaa said.
Medics said at least 187 protesters were wounded on Monday after a dramatic escalation in violence which began with a huge anti-government march on Sunday. At least 400 protesters and police have died since the revolt began eight months ago.
Protesters had been planning to ratchet up demonstrations this week. They said they expected a spike in bloodshed as they pushed their marches into areas surrounded by government troops in a bid to re-energise a languishing protest movement.
"We have known that this regime would kill its citizens," said Manea al-Mattari, from a leading council of youth activists. "But we know we have to do this, let our blood spill so the world notices how much Yemenis want their freedom."
The violence on Monday began when troops fired at an area seized by protesters on Sunday night to force them back to Change Square. A Reuters reporter saw snipers shooting from rooftops into the throng of demonstrators. Some of the deaths appeared to have been caused by rocket-propelled grenades.
Injured people were whisked on motorcycles to a mosque transformed into a makeshift hospital, where ambulances were arriving with shattered windows and pockmarked with bullet holes. Copies of the Koran were laid on the chests of the dead.
As shelling and gunfire continued in Sanaa, pro-opposition tribesmen said clashes had also surged in Arhab, a tribal area north of Sanaa, after they attacked a military base there.
Militants suspected of links to al Qaeda clashed with the army in the Abyan provincial capital of Zinjibar, just over a week after Yemen declared its troops had "liberated" the city from Islamist fighters.
Saadaldeen Talib, a former opposition parliamentarian, said he was concerned "complete disintegration and chaos might come very soon."
In Geneva on Monday, Yemeni Foreign Minister Abubakr al-Qirbi said Sunday's bloodshed would be investigated and perpetrators would be prosecuted.
In a speech to the U.N. Human Rights Council, he said: "The government of Yemen expresses its sorrow and condemnation for all acts of violence and bloodshed as those that happened yesterday in Sanaa. The government will investigate and hold accountable all those in charge of these acts."
Sanaa for months has been split between Mohsen's breakaway troops and Saleh loyalist forces in a maze of checkpoints, roadblocks and armoured vehicles.
Protesters on Monday managed to extend the territory of their camp by around one kilometre after hundreds slept there overnight. Mohsen's troops entered the area and were fortifying it with sandbags.
The new staked-out area brought protesters and troops backing them within 500 metres (1,650 feet) of the office of Ahmed Ali Saleh, the president's son and head of the Republican Guard units loyal to the government.
"I will go back out once the doctors check the wound," said Dhuyazen al-Shiah, 23, whose eye was bandaged after bullet fragments hit his face in Sunday's clashes.
"I do this because I was tired of living with no dignity. I worked as a smuggler through Saudi Arabia because I couldn't find a job here. I am committed to this now. I'll keep going and either succeed or I'll die."
Additional Reporting by Khaled al-Mahdy in Sanaa, Dhuyazen Mukhashaf in Aden, Stephanie Nebehay in Geneva, Isabel Coles and Angus McDowall in Dubai; Editing by Myra MacDonald