BANGKOK, Dec. 2 - Thailand's anti-government protest turned ugly Tuesday as a protester died in a grenade blast at Bangkok's domestic Don Muang airport, authorities said.

The grenade was fired into the inbound passenger terminal building of the airport, injuring 22 others, Bangkok Nation reported.

The facility, along with the international Suvarnabhumi airport, has been occupied by protesters led by People's Alliance for Democracy group demanding the resignation of Prime Minister Somchai Wongsawat's government, which is seen only as a proxy for former Prime Minister Thaksin Shinawatra, ousted in a military coup in 2006.

The Nation reported one protester, identified as a 29-year-old man, died en route to a hospital. Five victims were reported in serious condition and two critical.

PAD guards told the Nation they saw the attacker speed off on a vehicle.

On Sunday, a blast from a grenade suspected of being tossed by counter-demonstrators injured about 50 protesters.

Since the Sunday incident, the protesters have left the government compound after occupying it since August and are now concentrating their effort at the two airports, the Bangkok Post reported.

The move also was seen as designed to facilitate Friday's royal ceremonies for the 81st birthday of Thailand's highly revered King Bhumibol Adulyadej.

Separately, a constitutional court decision was awaited, which may force the prime minister to step down and dissolve his governing People Power Party and its coalition partners.

Tens of thousands of passengers have been stranded across the country since the airport protests. On Monday, the protesters allowed 37 of the 88 planes parked at Suvarnabhumi to fly out empty to pick up stranded passengers at other airports. (UPI)