An anti-government protester was killed and 22 others were wounded by a bomb blast at one of Bangkok’s two blockaded airports last night.

There were reports that a grenade was fired from a flyover near Don Muang domestic airport, which has been occupied since Thursday by royalist demonstrators from the People’s Alliance for Democracy (PAD).

The PAD, whose supporters wear yellow to show their backing for the King of Thailand, has described its mass sit-in as the “final battle” in its efforts to remove the Prime Minister, Somchai Wongsawat. Amid counter-demonstrations by the red-shirted activists of the ruling People Power Party, there have been fears of a civil war. Until yesterday, the protests had been largely peaceful, although there were accusations that a PPP supporter shot a PAD demonstrator near the main Suwaranabhumi international airport.

An emergency official said 17 of the wounded in yesterday’s blast, which took place last night, had already been discharged from hospital. Fears of violence escalated after hundreds of protesters of both factions began moving through the capital.

PAD activists abandoned their sit-in at the Prime Minister’s office to reinforce those at the two airports, while PPP supporters surrounded the constitutional court, which is due to rule today on claims that people were paid to vote for the party, which could be dissolved if the claims are upheld.

The court has moved with uncharacteristic speed to wrap up the case. If it finds against the PPP, Mr Somchai and other leaders would be barred from politics and many cabinet ministers would have to step down. The |dissolution of the PPP, however, will not necessarily mean a snap election, because many MPs will simply switch to a new “shell” party. The PAD |accuses Mr Somchai of being a pawn for his brother-in-law, the former prime minister Thaksin Shinawatra, who was ousted in a 2006 coup and is now in exile.

Mr Somchai insisted yesterday that he would not step down. “I will not quit and I will not dissolve parliament,” he said in the northern city of Chiang Mai.

The chaos has worried Thailand’s neighbours, who are due to meet in the country in two weeks for a regional summit. This may now be delayed.

Suwaranabhumi Airport has been blockaded for a week and stranded foreign travellers were growing more desperate yesterday with no sign of an end to the situation. All the main insurance companies have said they will not pick up the tab for accommodation and other costs, leaving many visitors to Thailand with money worries. Among Britons, there was anger that UK officials had done less for their nationals than most other countries. France, Spain, China are chartering aircraft to fly people out, while the Australian embassy is helping stranded tourists in Bangkok travel to the southern island of Phuket, where air traffic has not been disrupted.

The British embassy in Bangkok was closed all weekend and did not return calls yesterday. “Every other country is chartering aircraft to pick up their citizens; ours is doing nothing,” said Helen Coultish, 55, who was trying to get back to Britain with her husband John, 62. “I’ve got an elderly mother who thinks we are in the middle of riots. She is very worried.”