Thailand's strife-hit capital is bracing Monday for fresh opposition protests described as "judgement day" by demonstrators seeking to overthrow the embattled premier, despite her offer of early elections.

Prime Minister Yingluck Shinawatra has faced more than a month of 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".

Thai opposition lawmakers resigned en masse from parliament Sunday, deepening the kingdom's political crisis.

The protesters are united in their loathing for Yingluck's brother Thaksin Shinawatra, a billionaire telecoms tycoon turned populist politician who was ousted by royalist generals in a coup seven years ago.

His overthrow ushered in years of political turmoil and sometimes bloody street protests by the royalist "Yellow Shirts" and the rival pro-Thaksin "Red Shirts".

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

The unrest has left five people dead and more than 200 injured and the fear is that the fresh protests could bring fresh violence.

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.

With turnout dwindling, protest leader Suthep Thaugsuban has called for a final push on Monday to bring down the government.

"We want you to come out and march in every road. We will not go home empty-handed," he said in a speech to supporters late on Sunday, calling for the world's biggest-ever rally.

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.

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 Thaksin.

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

Pro-Thaksin parties have won every election in more than a decade, and Yingluck on Sunday 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 protest leaders, however, have said that they would not be satisfied with new elections.

The opposition Democrat Party -- which said Sunday its roughly 150 MPs were quitting because they could not achieve anything in parliament -- has not won an elected majority in about two decades.

New concrete barriers have been put in place around the government headquarters ahead of the planned protest, but unlike in previous demonstrations, 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 demonstrations were triggered by an amnesty bill, since dropped by Yingluck's ruling party, which opponents feared would have cleared the way for her brother 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.