News reports said the grenade landed near the prime minister's offices, which have been occupied by protestors led by the People's Alliance for Democracy (PAD) since August.

The blast occurred near at Government House where thousands of PAD supporters, who are demanding Prime Minister Somchai Wongsawatto step down, were holding a rally.

PAD leader Suriyasai Katasila told Channel 3 that he had come down from the stage about 30 minutes before the grenade dropped into a crowded area. The injured protestors were rushed to nearby hospitals, TV reports said.

He accused pro-government supporters of launching the attack. The explosion came hours after anti-government demonstrators forced police to abandon a checkpoint at the main Suvarnabhumi airport.

In the latest clash with protestors, about 150 riot police left their checkpoint near Suvarnabhumi International Airport on Saturday night after were attacked by PAD supports armed with iron rods.

The closures of Suvarnabhumi International Airport and Don Mueang domestic airport have trapped some 100,000 passengers.

The total number of the affected travelers could hit 300,000 as airport takeover continues, Tourism and Sports Minister Weerasak Kowsurat said.

The government said tourist industry was suffering and the number of visitors to Thailand could fall by half next year.

PAD supporters are seeking to topple the government, which they accuse the government of being on behalf of ousted and exiled Prime Minister Thaksin Shinawatra.

The situation in Bangkok was tense in Bangkok, especially at the two airports seized by anti-government protesters, after the government declared a state of emergency at the airports on Friday.

Meanwhile, Prime Minister Somchai is now in the northern province Chiang Mai, which he made it a temporary government seat after returning from an APEC meeting. Deputy Government Spokeswoman Suparat Nakboonnam earlier said that Somchai has no schedule to return to Bangkok in the near future.

She said Somchai decided to remain in Chiang Mai for security concern due to uncertainties related to the moves of the armed forces, and his moves would be kept secret.

In a related development, Thailand is considering to postpone the Association of Southeast Asian Nations (ASEAN) summit due to the political turmoil in the country, according to Thai Foreign Minister Sompong Amornwiwat.

Leaders of 10 ASEAN countries plus those of China, Japan and South Korea, are scheduled to meet in Chiang Mai between Dec. 14 to 18.

The summit was initially planned to be held in Bangkok, but the government decided to move to Chiang Mai because of the protestors.