* Prime minister vows to stop protesters
* Red shirts mass at new rally site
* Army has yet to establish perimeter around protesters
* Death toll rises to 24, at least 179 wounded
* U.S. embassy offers to evacuate non-essential personnel
(Recasts, adds new details throughout)
By Ambika Ahuja and Bill Tarrant
BANGKOK, May 15 (Reuters) - Prime Minister Abhisit Vejjajiva vowed on Saturday to stop protesters from toppling his government as fighting in Bangkok spiralled into chaotic urban warfare, with both sides calling for reinforcements.
Soldiers fired live rounds at demonstrators who fought back with petrol bombs, rocks and crude homemade rockets in two major areas of the city as the army tried to enforce a security cordon around a sprawling protest encampment in central Bangkok.
"We will not retreat," Prime Minister Abhisit Vejjajiva said in a televised statement. "We cannot allow the country to be in a condition in which people can establish an armed group to topple the government that they are not happy with."
"Red shirt" protestors, made up of the rural and urban poor, accuse Vejjajiva and his royalist urban backers of running the southeast Asian country with impunity by meddling in the judicial system and toppling elected governments. [nSGE64C063]
The protestors, who have adopted red as a protest colour and broadly support former premier Thaksin Shinawatra, set fire to vehicles and hurled rocks at troops who set up razor wire across deserted roads in the business district and fired back.
A sign at one intersection warned residents not to enter a "live bullet area". Another at a separate site warned of a "rubber bullet area". Both were later taken down.
For full coverage, click on [nTHAILAND]
By nightfall, at least 2,000 protesters had massed around an intersection in the working-class Klong Toey district, using a truck as a makeshift stage for protest leaders, in a possible move toward setting up a separate sit-in site.
Residents were asked to show identification to prevent people from joining the crowd.
Red shirt leader Nattawut Saikua told thousands still hunkered down in their main encampment that reinforcements were coming. "We have been contacted by leaders in several provinces that they will mobilise to help us pressure the government."
Witnesses described the fighting as one-sided, as troops armed with automatic rifles easily dodged projectiles and opened fire. Soldiers can shoot if protesters come within 36 metres (120 ft), said army spokesman Sansern Kaewkamnerd, adding more soldiers were needed to establish control.
"I cannot say how many troops are deployed due to security concerns, but there will be reinforcements to help troops seal the area and step up pressure on protesters," he said.
The Public Health Ministry said at least 24 people had been killed and 179 wounded since fighting began Thursday night with the shooting of a renegade general allied with the protesters.
The toll was expected to rise sharply, as fighting continued in two areas of the city of 15 million people. Power has been cut in those areas.
Six M-79 grenades fired at a police housing compound near the encampment on Saturday night wounded at least six people, a police spokesman said.
The U.S. Embassy offered to evacuate families and partners of U.S. government staff based in Bangkok on a voluntary basis, and urged its citizens against any travel to Bangkok. [nSGE64E033]
"The troops may be making some progress on sealing the area but at a great cost," said Thitinan Pongsudhirak, political scientist at Chulalongkorn University, adding rising casualties could weaken Prime Minister Abhisit Vejjajiva.
"Is the government successfully dispersing the crowd and progressing toward ending the crisis? The answer is no, not so far, and it’s a long way to go."
The army is battling to set up a perimeter around the 3.5 sq-km (1.2 sq-mile) encampment where at least 5,000 people are hunkered down, including women and children, behind barricades made of tyres, poles and concrete, topped by razor wire.
The crisis has paralysed Bangkok, squeezed southeast Asia’s second-biggest economy, scared off tourists and choked investment in one of Asia’s most promising emerging markets.[ID:nSGE63604Y]
"My ears are ringing with all the shooting last night," said Ratana Veerasawat, a 48-year-old owner of a hole-in-the-wall grocery store north of the protest encampment where many residents were leaving for safer locations.
U.N. Secretary-General Ban Ki-Moon expressed concern over "the rapidly mounting tensions and violence", and called for resumption of dialogue between the two sides. [nN14236286]
Protesters remained defiant, despite evidence that the government’s strategy of starving them out of their encampment was having an effect.
Supplies of food, water and fuel were starting to run thin as the red shirt delivery trucks were being blocked, said one protest leader, Kwanchai Praipanabut, adding they still had enough to hold out for day.
Some red shirt leaders, including the movement’s chairman, haven’t been seen for days. Those at the site wore flak jackets, fearing snipers. They all face criminal charges.
"I am not scared," said Sanae Promman, a 37-year-old protester frying vegetables in a wok under a tent at the site. "Some of my friends have left because they are scared but many are still here to fight. We will fight until we die if we must."
The fighting is the latest eruption in a five-year crisis between the rural poor and urban working classes, who accuse an "establishment elite" -- comprising royalists, big business and military brass -- of colluding to bring down two elected governments.
Those governments were led or backed by Thaksin, who lives abroad to avoid imprisonment on a graft conviction.
The red shirts and their supporters say the politically powerful military influenced a 2008 parliamentary vote, which took place after a pro-Thaksin party was dissolved, to ensure the British-born, Oxford-educated Abhisit rose to power.
(Additional reporting by Martin Petty, Jerry Lampen, Panarat Thepgumpanat, Ploy Ten Kate, Chalathip Thirasoonthrakul and Thin Lei Win; Editing by Reed Stevenson)