BANGKOK—A grenade attack on demonstrators occupying the Thai premier's offices killed one person and wounded 29 Thursday, prompting protest leaders to call for a new march against the government.

In the first fatal assault inside the Government House compound since the People's Alliance for Democracy (PAD) stormed it in August, unknown attackers fired the grenade into the compound before dawn Thursday, Bangkok police said.

PAD chiefs accused the government of masterminding the attack and called for massive protests against the administration, which they allege is a corrupt proxy for ousted prime minister Thaksin Shinawatra.

"The PAD will no longer tolerate daily brutal crackdowns by the government and cannot accept any type of Thaksin regime," the group said in a statement.

"Therefore we have reached a consensus to call for a mass protest on Sunday November 23 at 2:00 pm (0700 GMT) to move to parliament to finally get rid of the proxy government and stop the tyrants."

About 300 PAD supporters brought the dead man's body in the back of a pick-up truck to Bangkok's Metropolitan Police Headquarters to call for justice, a policeman there said, before taking the coffin to a nearby temple.

The 48-year-old man died in hospital of shrapnel wounds, an emergency services spokeswoman said. Most of the 29 people injured have been discharged, she said, but two remain in a critical condition.

Prime Minister Somchai Wongsawat, who is Thaksin's brother-in-law, immediately denied any involvement and vowed a swift police investigation into the incident.

"It is not government policy to use force or violence," Somchai told reporters. "No one wants bloodshed or killing among Thais."

Thailand has been gripped by political turmoil since May when the PAD began their rallies, but there was a respite for the six-day funeral of Princess Galyani, late sister of deeply-revered King Bhumibol Adulyadej.

The elaborate cremation ceremonies ended Wednesday evening and within hours the blast hit in front of a stage at the sprawling Bangkok protest site, where hundreds of people are camped out in a fortified tented city.

"At first I heard a loud bang and I thought it was a firecracker, then I saw I was bleeding," said a 68-year-old woman who had been sleeping near the blast site and sustained cuts to her hand.

Chamlong Srimuang, a PAD founder, vowed to bring supporters to parliament on Sunday ahead of a joint session on Monday for a "last fight".

"We have demonstrated for 180 days," he told reporters. "We will fight as best as we can and we hope we will win."

Somchai urged protesters to continue their demonstration "within the law" but said the police and army had a series of steps to follow if the situation spiralled.

Meanwhile, army chief Anupong Paochinda said the army would be ready to assist police if needed.

The months-long stand-off last erupted into bloodshed on October 7 when police fired tear gas at demonstrators trying to block parliament, sparking clashes that left two people dead and nearly 500 injured.

National Security Council (NSC) chief Surapon Peunaiyaka told reporters Thursday a repeat of violence was unlikely.

"I am worried (about Sunday) but I think police have learnt lessons from what happened on October 7 and they will change their conduct accordingly," he said after the monthly NSC meeting.

Police refused to speculate on who was behind Thursday's attack, but General Jongrak Chutanont, deputy national police chief, said the explosion was caused by a grenade fired from an M79 launcher outside the compound.

He said police had set up checkpoints across the city.

"This is in response to the rumors that there will be a series of bomb attacks in the city after the royal cremation," he said.

A number of small blasts have rocked the PAD protest camps in the weeks since the bloody street battles, injuring several people.

Police meanwhile have said the PAD's personal militia who guard the protest camp -- known as the "Srivijaya Warriors" -- are armed, and two people have been shot after rows near the occupied compound in central Bangkok.

Thaksin, who was ousted in a military coup in 2006 and is living in exile to avoid corruption charges, is set to make a telephone address to his supporters next month, which could further inflame the situation.

Thaksin's allies won elections in December, angering the old power cliques in the palace, military and bureaucracy who tried to rid Thailand of Thaksin's influence.