BANGKOK, Oct 7, 2008 (AFP) - Police fired tear gas Tuesday on protesters massing outside parliament in the Thai capital, injuring 65 people as months of political turmoil boiled over, police and a medical official said.

Seven people were critically injured in the police crackdown, the official said, after thousands of anti-government protesters besieged the building to try to stop a key speech by the new prime minister going ahead Tuesday.

'Police have had to disperse the protesters by firing tear gas to make way for MPs to enter parliament,' Major General Anan Srihiran of the Metropolitan police told AFP.

Nanthana Mesprasart, supervisor at Bangkok's emergency medical response centre, told AFP the 65 injured had been taken to three hospitals for treatment, and one man had lost his left foot in the chaos.

Thousands of supporters of the People's Alliance for Democracy (PAD) marched to parliament late Monday, using trucks to try and seal off the building where the premier was due to deliver the government's first policy address.

The protesters moved from Bangkok's main government compound nearby where they have been camped since late August, after lawmakers agreed in principle to amend the constitution -- a move the PAD opposes.

About 1,000 police officers used tear gas and smoke bombs against 2,000 protesters to try and retake control of a road in front of parliament, a police official who did not want to be named told AFP

Witnesses said MPs had arrived for the parliamentary session, but it was unclear if it would be able to go ahead.

Protesters also cut the power to the parliament building, before emergency generators kicked in after a short black-out, an AFP reporter said.

On Tuesday television stations showed images of demonstrators trying to barricade themselves in around Bangkok's parliament building with piles of tyres, while ambulances rushed to the scene.

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 Wongsawat formed his new government last month.

Somchai urged the PAD to leave his offices by November, and appointed his deputy Chavalit Yongchaiyudh to negotiate with their leaders.

Those talks were jeopardised over the weekend with the arrest of key PAD head Chamlong Srimuang and protest organiser Chaiwat Sinsuwong.

Protest leader Sondhi Limthongkul told the crowds who remained at Government House late Monday that the march on parliament would be their 'final battle', but Somchai insisted the house session would go ahead.

'It is not right for the PAD to obstruct the representatives of the people,' he said on Monday.