Members of the People's Alliance for Democracy members broke through police lines and began roaming through the sprawling US$4 billion (S$6.03 billion) terminal as startled tourists looked on.

Earlier, anti-government supporters clashed with guns, knives and slingshots, in the latest sign that the country's long-running political crisis may be veering toward open violence in the streets.

The fighting occurred as anti-government demonstrators were disrupting traffic on another highway on the other side of town, delaying travelers to and from the city's main airport.

The clash began when government supporters threw rocks at a truck carrying members of the People's Alliance for Democracy along a busy highway.

The alliance members were returning from a rally at Don Muang airport, which they had been besieging since Monday in an effort to stop the government from meeting there. The prime minister's office, where meetings are normally held, has been occupied by the protesters since late August.

The members of the anti-government alliance - wearing yellow arm bands and camouflage clothing - responded by firing slingshots and a half-dozen shots with two pistols from their truck, according to footage shown on Thai PBS television.

They also hoisted a portrait of King Bhumibol Adulyadej, Thailand's revered monarch, whose interests they claim to be working for.

The footage showed the alliance supporters surrounding a motorcycle taxi driver and putting a knife to his throat. After the driver fled, the protesters battered several motorbikes with steel rods and set fire to another one.

The clash was the second time in recent months that the two sides have fought and marks the first major violence since Oct 7, when street battles with police and anti-government forces left two people dead and hundreds wounded.

In a Sept 2 clash between the two sides, a government supporter was beaten to death, while two alliance members were killed last week in grenade attacks.

The rivals are fairly easy to distinguish, since the protest alliance favors yellow shirts and their rivals red ones. Some of the government supporters seen fleeing were wearing red shirts.

Police Col. Piyapong Ponvanich said 11 people were wounded in the clash, most of them government supporters, some with gunshot wounds.

Government spokesman Nattawut Sai-gua said authorities would beef up security, but that it was premature to talk about declaring a state of emergency.

'The police have beefed up security all over the city after what happened. They have set up 53 checkpoints on the streets of Bangkok in case of more chaos of this nature on the street. The situation is still under control,' he said.

The pro-government crowd appeared to be members of a loose association of taxi and motorcycle drivers who support the government and are supporters of former Prime Minister Thaksin Shinawatra.

Protesters elsewhere used trucks to block part of the expressway to Thailand's main international airport, tying up traffic ahead of the planned Wednesday return by Prime Minister Somchai Wongsawat from Peru, where he attended a summit of Pacific Rim leaders.

They have blocked all but one lane of the road to the airport, the main hub for millions of tourists who visit South-east Asia.

The alliance opposes Mr Somchai's government because it considers it to be a puppet of Thaksin, who was accused of corruption and abuse of power and deposed in a September 2006 military coup.

Tuesday's clash came a day after thousands of anti-government supporters besieged Parliament and then Don Muang airport in a bid to shut down the government.

On Monday, police yielded as demonstrators pushed past them, even though the protesters sometimes showered them with expletives in an apparent effort to provoke a violent response to discredit the authorities.

The government evidently sought to turn the tables on the protesters, who in past actions managed to attract public sympathy with claims of police brutality.

Protesters seeking Mr Somchai's resignation have occupied his Bangkok headquarters, known as Government House, since Aug 26, forcing him to relocate to a makeshift office in the VIP area of the former international airport at Don Muang.

A strike called by state enterprise unions to disrupt air, road and rail service in support of the protest fizzled - the second time unionists have failed to deliver on promises of a crippling work stoppage.

Thailand's political crisis began in 2006, when a similar campaign against Thaksin led to a him being deposed by the coup. But further efforts to cripple Thaksin?s political machine failed, and his allies won a December 2007 election.

The alliance then resumed its street protests and finally stormed Government House in August, vowing not to leave until they have forced Thaksin's allies from power. They accuse Mr Somchai of acting as a proxy for Thaksin, who is his brother-in-law.

Thailand's economy, already struggling amid the global downturn, has been hit hard by the political turmoil. The state planning agency said on Monday it grew at its slowest pace in more than three years this past quarter. -- AP, REUTERS.