Thailand's opposition MPs have resigned en masse from parliament, deepening the kingdom's political crisis as anti-government protesters vowed a final showdown despite an election offer from the embattled prime minister.

Bangkok is bracing for another major opposition demonstration on Monday aimed at toppling Yingluck Shinawatra and curbing the political influence of her brother Thaksin.

The kingdom has been rocked by several episodes of political bloodshed since Mr Thaksin, a billionaire tycoon-turned-premier, was ousted by royalist generals in a coup seven years ago.

Ms Yingluck's government has been shaken by more than a month of rolling rallies by demonstrators, sometimes numbering in their tens of thousands, who want to suspend the country's democracy in favour of an unelected "People's Council".

The opposition Democrat Party says its 153 MPs were resigning from the 500-seat lower house - a move that does not prevent Ms Yingluck's Puea Thai party from passing new laws, but which leaves parliament facing questions about its legitimacy.

"We performed our duty in parliament as best we could," said Democrat leader and former premier Abhisit Vejjajiva.

"We cannot do any more. We regret that the majority in parliament betrays the voice of the people."

The move came soon after Ms Yingluck renewed her offer of elections if the protesters - a mix of royalists, middle class Thais and other Thaksin opponents - agree to respect the democratic process.

"The government is ready to dissolve the house if the majority wants it," she said in a televised address, noting that under the kingdom's laws an election would have to be held within 60 days.

But she added: "If protesters or a major political party do not accept that or do not accept the result of the election, it will just prolong the conflict."

She also floated the idea of a referendum to solve the crisis, but it was unclear what the nation would be asked to vote on.

'There must be a solution'

Protest leaders have said they would not be satisfied with new elections, but the opposition Democrats hinted it might take part in any new polls, even though they have not won an elected majority in about two decades.

"House dissolution is one way of returning power to the people. But there must be a solution to make people confident in the election," Mr Abhisit said.

Anti-government protesters rally at Bangkok's government complex. ( AFP Photo: Pornchai Kittiwongsakul )

Thailand's political conflict broadly pits a Bangkok-based middle class and royalist elite backed by the military against rural and working-class voters loyal to Mr Thaksin.

Pro-Thaksin parties have won every election in more than a decade.

The former premier went into exile in 2008 to avoid jail for a corruption conviction, which he says was politically motivated.

Tensions remain high in the kingdom following several days of street clashes last week between police using tear gas, water cannon and rubber bullets against rock-throwing demonstrators.

The unrest has left five people dead and more than 200 injured in Bangkok.

Demonstrators and police have observed a temporary truce since Wednesday for the 86th birthday of King Bhumibol Adulyadej, who is treated as a near-deity by many Thais.

Protest leader says Monday is 'judgement day'

With turnout dwindling, protest leader Suthep Thaugsuban has called for a final push on Monday to bring down the government, describing it as "judgement day".

The former deputy premier, who now faces an arrest warrant for insurrection, has vowed to surrender to the authorities unless enough people join the march to the government headquarters.

New concrete barriers have been put in place around the seat of government ahead of the planned protest, but unlike previously security officials said barbed wire would not be used.

"The police will keep up negotiations and to try avoid any injury or death," said national police spokesman Piya Utayo, urging protesters to respect the law.

The government's "Red Shirt" supporters are planning their own rally on Tuesday in the ancient capital of Ayutthaya north of Bangkok.

The recent protests were triggered by an amnesty bill, since dropped by Ms Yingluck's ruling party, which opponents feared would have cleared the way for Mr Thaksin's return.

They are the biggest and deadliest street demonstrations since 2010, when dozens of people were killed in a military crackdown on mass pro-Thaksin Red Shirt rallies in Bangkok.

AFP