The protesters have occupied the Government House compound since August and are demanding that Prime Minister Somchai Wongsawat resign. They accuse him of being a proxy for his brother-in-law, former Prime Minister Thaksin Shinawatra, who was ousted by a 2006 military coup for alleged corruption and abuse of power.

The protest group, which calls itself the People's Alliance for Democracy, has been attacked several times by small bombs and grenades, including a blast Thursday that killed one person and wounded 29, and another on Saturday that injured eight.

The alliance plans a mass demonstration on Sunday in a final push to 'finish off' the government, key protest leader Chamlong Srimuang said.

The protesters are expected to march to Parliament in an attempt to block a Monday session of lawmakers.

'It will be the biggest PAD movement to finish off this illegal government,' Chamlong told about 2,000 protesters at Government House on Saturday. 'If we cannot drive out this illegal government then we will give up and let them do whatever they want to the country. Everyone in PAD will go home.'

Army spokesman Colonel Sansern Khaewkamnerd said the demonstration could turn violent if the protesters march to Parliament.

The army urged the government to postpone or change the site of the parliamentary session but it refused, Col Sansern said, adding that the army will station about 2,000 soldiers in the area as a precaution.

'Police will be responsible for control of the situation but the military will be on call to support police in keeping order,' Col Sansern said in a telephone interview.

Bangkok police chief Gen. Jongrak Jutanon said 2,400 police would be stationed outside Parliament.

'I hope the protesters do not obstruct members of Parliament from doing their duty,' he said on Saturday.

The last time the protest group marched on Parliament, street battles with police left two dead and hundreds wounded. The Oct 7 clashes were the country's worst political violence in more than a decade.

No one took responsibility for Saturday's explosion, which occurred shortly after 2 am Police Lt. Col. Samreng Songsiang said it was caused by an M-79 grenade, a type normally fired from a launcher.

He said it came from somewhere nearby but could not pinpoint the location.

The Nation newspaper reported that witnesses saw two teenagers park a motorcycle near the back of the nearby headquarters of the Bangkok Metropolitan Police and fire the grenade from a launcher.

Mr Chamlong, however, said the grenade was launched from the police headquarters itself, but gave no evidence to support the claim.

Thaksin has fled Thailand to escape a slew of corruption charges.

He remains highly popular among poor Thais living in the countryside for his populist policies, but is fiercely opposed by the protest alliance and its sympathizers - monarchists, the military and the urban elite - who viewed his government as deeply corrupt. -- AP