Bangkok - Thai soldiers fired at protestors driving buses at them and used tear gas against demonstrators blocking a main road junction in Bangkok Monday in a major escalation of political violence in the kingdom.

   Medical authorities said scores of people had been treated for injuries, after troops swept through road junctions around Victory Monument, which had been occupied by anti-government protestors.

   A leader of the so-called red shirts, Nattawut Saikua, called on demonstrators to fall back on their base area in front of Government House following much more determined military action.

   The armed forces supreme commander, General Songkitti Chakkrabat, warned in a late afternoon television broadcast that troops would use "all possible measures" to restore order and peace to the capital.

   Battle-ready soldiers, many of them brought in from the Cambodian border, fired into the air and anti-riot police fired tear gas in an effort to disperse the protesters.

   Some of the protestors threw Molotov cocktails at the troops and set several buses on fire, but most withdrew as soldiers advanced.

   Army troops started moving into the central Din Daeng area early Monday in what appeared to be a slow but firm start to the anticipated crackdown against thousands of red-shirted protestors loyal to former prime minister Thaksin Shinawatra who had shut down much of central Bangkok.

   The clashes started before dawn, some distance from the centre of the main anti-government demonstration in the streets around Government House, the office of Prime Minister Abhisit Vejjajiva.

   Much of the city was shut down for the Thai New Year holiday of Songkran, but the violence also kept cars off the streets and several of the main shopping centres were closed and intersections blocked with commandeered buses.

   Abhisit, in an afternoon television address, urged all protestors to go home, especially those camped around Government House. The prime minister said about 70 people had been hurt, one-third of them soldiers. No-one had died, he stressed.

   Earlier, troops backed by water cannon marched towards protestors. When a bold protestor reversed a city bus towards the lines of advancing soldiers they unleashed a tremendous fusillade - stopping the bus.

   The military - under great pressure from the government to finally act decisively - was attempting to clear the streets of taunting protestors, many on motorbikes

   The 10,000 or so demonstrators at a main protest rally at Government House, however, remained a more daunting challenge. Dozens more buses have been used to block roads and ferry protestors around the city.

   Other troops were moving to clear protestors out of the Foreign Ministry grounds, said witnesses. Officers said they were under orders to clear the streets as quickly as possible.

   Colonel Sansern Kaewkamnerd, the army spokesman, said troops in the morning first tried to negotiate with the protestors. The talks broke down after some protestors tried to ram buses against the soldiers, troops at the scene said.

   Police said about 30,000 protestors had seized many road junctions near the seat of government, commandeered several city buses and erected makeshift barricades.

   A government announcement on television in the early afternoon said soldiers were moving to secure airports, ports and strategic infrastructure so that no one need worry about being cut off in Thailand.

   Nevertheless, several governments issued travel advisories against visiting the kingdom.

   By mid-afternoon the troops still had not moved in force against the main red-shirt roadblocks around Government House which were manned by determined protestors, many carrying clubs or iron bars, witnesses said.

   Some protestors at roadblocks had stacks of Molotov cocktails on hand. "We are not going to move. What are they going to do? Kill us all?" said Chaiya Chaiwichayakul, 33, a stonemason, at a roadblock about 500 metres from Government House.

   Abhisit declared a state of emergency in Bangkok and surrounding provinces on Sunday in a bid to quell the protests that forced the cancellation of a major Asian summit meeting the previous day.

   The government was widely criticized Sunday for failing to crack down on the relatively small number of protestors who invaded the 16- nation summit of the Association of South-East Asian Nations (ASEAN) at the seaside resort of Pattaya, south-east of Bangkok, over the weekend, forcing it to be postponed.

   Thailand has been roiled by political strife since the overthrow of Thaksin in September 2006 in a bloodless military coup. Two subsequent pro-Thaksin governments were rejected by the military and bureaucracy amid widespread demonstrations, leading to the formation of Abhisit's Democrat Party-led coalition government four months ago.

   Thaksin is popular especially with the rural poor because of his populist economic policies, but is loathed and distrusted by much of the military, bureaucracy and middle class, who accuse him of corruption and disloyalty to the monarchy.

   Thaksin, who has spent recent weeks whipping up support for "a revolution against an illegal government," has lived abroad after being sentenced to two years in jail for abusing his powers.