Bangkok - Thousands of followers of the People's Alliance for Democracy (PAD) surrounded Parliament Tuesday, forcing the prime minister to flee the compound by helicopter, after police failed to disperse the anti-government protestors with tear gas.

Riot police attempted to scatter the demonstration around Parliament early Tuesday by firing tear gas into the crowd, injuring about 70 people, several seriously.

At least two protestors lost their lower legs and another lost his right hand, apparently from tear gas canisters either shot at close range or detonated when the victims attempted to kick or throw them back at police.

"We want the government to dissolve parliament by 6 pm," said Sonthi Limthongkul, a key leader of the PAD which has occupied Government House, the seat of government, since August 26. "If not we will take strong measures against the government."

Thousands of PAD followers marched on Parliament Monday night in an effort to prevent the lower and upper houses from holding a session to approve the new government's policy statement.

Although the PAD demonstration failed to prevent the parliamentary session from being held, they succeeded in surrounding the compound and locking the gates, blocking some with police vans and trucks whose tyres had been flattened.

Prime Minister Somchai Wongsawat, after reading his government's policy statement, was forced to flee the Parliament compound in a helicopter. Hundreds of legislators were trapped within the compound with no way out by car.

After the initial attack on the PAD at about 6:20 am, demonstrators stormed the Metropolitan Police headquarters on Rajdamnoen Nok Road, around the corner from Parliament, to protest the crackdown.

"I was just sitting there. I didn't think the police would fire tear gas at us. This is barbaric," said Pradap Thaephu, 53, a woman PAD supporter who sustained a head wound from a tear gas cannister.

Thai Queen Sirikit has reportedly donated funds for medical treatment of the injured. The police attack immediately drew criticism from human rights groups and the opposition Democrat Party refused to attend the Parliament session to protest the use of violence.

The initial police attack left about 70 PAD followers injured, primarily from tear gas canisters but others from what appeared to be rubber bullet wounds, said medics who had set up emergency treatment stations in Parliament.

The police have denied using rubber bullets, but eyewitnesses said they saw guns being fired. Rubber bullets were also found on the street in front of Parliament.

Despite the police crackdown, the PAD protestors, many of them in tears from the gas, refused to leave the road in front of the Parliament building. Some of the protestors had armed themselves with golf clubs, iron bars and wooden sticks.

The PAD is a loose coalition of groups opposed to the return to power of former premier Thaksin Shinawatra, who was ousted by a military coup on September 19, 2006.

The current government is led by the People Power Party (PPP), a group with close ties to Thaksin that is now led by Somchai, Thaksin's brother-in-law.

There are fears among the PAD that this government is seeking to reinstate the 1997 constitution, a liberal, pro-political parties charter that paved the way for Thaksin's rise to power in 2001 to 2006 and allowed his Thai Rak Thai party to dominate the political scene unhampered by independent bodies and checks and balances.

A previous attempt by former premier Samak Sundaravej to amend the 2007 constitution, which favours the bureaucracy and has strengthened the judiciary and other checks on corrupt politicians, brought the PAD to the streets in May.

Samak lost his post last month when the Constitution Court found him guilty of moonlighting on the premiership for hosting television cooking shows.

The PAD protests culminated in the seizure of Government House on August 26. They have occupied the seat of administration since.

The arrest of PAD leader Chamlong Srimuang on treason charges Sunday was deemed an invitation to escalate the PAD protest and the derailing of efforts to reach a compromise with the movement that is fighting for an era of "new politics" in Thailand.