BANGKOK - Thousands of anti-government protesters marched to Thailand's parliament Monday, using trucks to try and seal off the building ahead of an address by the new cabinet, police and Thai media said.

Thailand's new ministers are due on Tuesday to deliver their first policy address to the house, but the so-called People's Alliance for Democracy (PAD) are demanding the elected government step down.

Protesters have occupied Bangkok's main government compound since late August, but many moved to nearby parliament Monday evening after lawmakers agreed in principle to amend the constitution -- a move the PAD opposes.

"Don't let the arrests of Chamlong and Chaiwat be wasted," he said, referring to the detention over the weekend of key PAD leader Chamlong Srimuang and protest organiser Chaiwat Sinsuwong.

An official from Bangkok police told AFP that about 4,000 people had gathered at parliament, while 6,000 were at Government House.

Thai television channels said that the PAD had blocked the entrances to parliament with trucks and showed images of protesters unfurling barbed wire, but new Prime Minister Somchai Wongsawat said the house session would go ahead.

"The house speaker has already made the appointment -- there will be no change. It is the duty of police to maintain law and order and make the way for lawmakers to enter parliament," he told reporters.

"It is not right for the PAD to obstruct the representatives of the people," he added.

PAD leaders are demanding that the elected government step down because of its links to ousted prime minister Thaksin Shinawatra.

The group began their campaign in late May claiming that the ruling People Power Party's plan to amend the constitution was aimed at helping Thaksin escape graft charges, and they stormed Government House on August 26.

Former prime minister Samak Sundaravej was forced from office weeks later after a court ruled he had accepted illegal payments for a TV show, and his successor Somchai formed his new government last month.

They are due to meet Tuesday to discuss their policy platform at Thailand's old airport -- which has temporarily replaced the besieged Government House -- before addressing parliament.

Somchai has made clear his intention to amend the constitution, and on Monday his People Power Party, its five coalition partners and the opposition Democrat Party agreed to support a process to change the charter.

The current constitution was drafted and brought in during military rule following the September 2006 coup.

Chai Chidchob, parliament spokesman, told reporters that charter amendments were needed to resolve political turmoil and said a constitution review body would be set up.

"Every party will send its representatives to work together in a working group, and that group will select (constitution) drafters later," he added.

The announcement appeared to anger the PAD.

"The PAD asks agreement from the supporters to extend their protest to parliament to respond to the government's attempt to amend the constitution," the group said in a statement.

"The details of the amendment aim to wipe out Thaksin's wrongdoing."

Thaksin has applied for political asylum in Britain, where he has spent most of his time since being forced out of office two years ago, officials said on Monday.