SINGAPORE Airlines said it will cancel all flights to and from Bangkok on Wednesday, Bloomberg news reported.

The carrier is canceling the services 'due to the deteriorating security situation, and a lack of operational service staff at Suvarnabhumi airport,' it said in an e-mailed statement on Wednesday.

Thai authorities warned of flight chaos for thousands of passengers and damage to the tourism industry after anti-government protesters stormed the main terminal at Bangkok's international airport, closing it down.

Four people were injured by a grenade this morning at the airport, TPBS television station reported. Parnthep Pongpourpan, a spokesman for the protesters, said the injuries weren't serious and the People's Alliance for Democracy group will wait for the return of Prime Minister Somchai Wongsawat, whose resignation they are demanding.

The demonstrators, who want Somchai to take responsibility for deadly clashes with police last month, may force him to declare a state of emergency to prevent escalating violence. The prime minister, set to return today from the Asia-Pacific Economic Cooperation (Apec) forum in Peru, has rejected the resignation calls and police have avoided using force since the Oct 7 clash in which two people died and 470 were injured.

'Tens of thousands of tourists will be stranded here as we stopped departure flights,' Porntip Hirunkate, secretary-general of the Tourism Council of Thailand, told Thai PBS television late yesterday. 'This will hurt our tourism in December, which is our high season. The impact may go further to next year too.'

Thousands of passengers slept overnight at the airport, many complaining airport and airline staff had disappeared when the PAD demonstrators, dressed in their movement's yellow shirts, invaded the airport late on Tuesday.

'We came here and we saw all these people in yellow. We thought they were football fans. Now we're just waiting,' said a Dutchman who gave his name as Mark.

Suvarnabhumi is one of Asia's busiest airports and gateway for nearly 15 million visitors to Thailand each year.

Police did nothing to prevent the occupation by the protesters, passengers said, and barely any officers were visible early on Wednesday.

American Kevin Harris said he had arrived at the airport for an early morning flight only to find people sleeping on baggage conveyor belts and benches all over the terminal.

'I just want to get home for Thanksgiving, but it's not going to happen. We have no idea what's happening here.'

'This isn't the fault of the airport but it's their responsibility to deal with it,' he added.

No flights had left since around 4am (2100 GMT on Tuesday, 5am Singapore time), people at the airport said. Some staff were present at the airport information desk at 7am on Wednesday.

Half the flights on information boards had 'cancelled' beside them but others were not leaving at their scheduled time.

One target of the PAD demonstrators was the flight bringing Mr Somchai home from a foreign trip.

Mr Somchai was due to return on Wednesday from an Apec summit in Peru but a government spokesman said he would not land at Suvarnabhumi. The Nation newspaper said on its website his plane would land at the northern city of Chiang Mai and then he would fly to Bangkok to chair a special cabinet meeting.

'I will get off the plane wherever it lands,' the Bangkok Post quoted Mr Somchai as saying from Peru.

Gunfire had broken out on the streets of the Thai capital on Tuesday as armed PAD members opened fire on government supporters. At least 11 people were hurt, officials said.

The alliance accuses Mr Somchai of being a puppet of ousted prime minister Thaksin Shinawatra, his brother-in-law. Thaksin, who was accused of corruption and authoritarianism while in office, is living overseas after skipping bail to avoid graft charges following his overthrow by the army in 2006.

The movement has the backing of Bangkok's urban middle classes and elite, while Thaksin and the government have the support of rural voters and the urban poor.

Worsening bloodshed could provoke another coup but army chief General Anupong Paochinda said on Tuesday that military intervention would not resolve the fundamental political rifts. -- REUTERS