A grenade blast has wounded more than 50 anti-government protesters in Bangkok ahead of a pro government rally later today.

The blast occurred around midnight at Government House, which members of the People's Alliance for Democracy (PAD) have occupied since August, in their bid to topple the government of Prime Minister Somchai Wongsawat.

PAD supporters have also taken over both of Bangkok's main airports, forcing them to close and stranding thousands of tourists, including at least 2000 Britons.

Last night, the militants, wielding slingshots and clubs, forced riot police to flee the main Suvarnabhumi airport, the gateway to Bangkok for British Airways and other international carriers.

By early afternoon today, police still had not returned to the airport.

Meanwhile thousands of desperate toruists were attempting to leave the country by whatever means possible.

Many have travelled to U-Tapao, a naval air-base at U-Tapao, 90 miles southeast of Bangkok. But the airport is in chaos with 3,000 people waiting to use its two x-ray machines and four check-in counters.

Last night's grenade attack was among the most bloody in the battle between anti and pro government supporters. A PAD spokesman said 51 people were wounded in the attack, with four in a critical condition.

It has provoked fears of violence at today's rally by the pro government Democratic Alliance Against Dictatorship (DAAD) which claims it will bring 100,000 supporters to central Bangkok in a show of support for the government.

The sit-ins at Suvarnabhumi and the city's domestic hub Don Muang are the latest escalation in the PAD's "final battle" to unseat the government.

The airport closures have crippled the tourism industry during the peak end-of-year season.

Deputy Prime Minister Olarn Chaipravat said the damage to Thailand's image may cut arrivals by half to 6-7 million in 2009, and threaten a million jobs.

He said it could take a month to fly all the tourists home.