Thailand's main opposition party has announced that its Members of Parliament would resign en masse, deepening the kingdom's political crisis, as anti-government protesters prepare for another major rally.

Abhisit Vejjajiva, Democrat party leader and former prime minister, told reporters the resignations were effective immediately.

Abhisit said Prime Minister Yingluck Shinawatra's government has been "illegitimate" ever since her ruling party tried to ram through an amnesty bill that critics allege was mainly designed to bring back Thaksin from exile. 

"The solution to our current problems needs to start with the showing of responsibility," Abhisit said. "The prime minister has never showed any responsibility or conscience.''

Prime Minister Yingluck Shinawatra has faced more than a month of opposition-backed demonstrations seeking to suspend democracy in favour of an unelected "People's Council".

The mass resignation is a largely symbolic move as parliament and government can continue to operate, Al Jazeera's Wayne Hay, reporting from Bangkok, said.

"But it is designed clearly to increase pressure on the government," he said.

Protesters have vowed a final showdown on Monday in efforts to topple Yingluck's government and curb the political influence of her brother Thaksin.

"We decided to quit as MPs to march with the people against the Thaksin regime," Democrat Party politician Sirichok Sopha said in televised remarks.

Thaksin lives in Dubai to avoid a jail sentence for a corruption conviction he says was politically motivated.

Opposition politicians and demonstrators were also reportedly seeking support from the military to topple Yingluck's government.

Earlier this year, nine opposition MPs resigned to lead the mass protests.

Yingluck said on Sunday she was willing to call an election to end the political crisis gripping the country -- but only if protesters seeking her overthrow accept the result.

As this developed, Al Jazeera's Veronica Pedrosa, reporting from Bangkok, said that a top security official has claimed that some protesters are plainclothes soldiers.

Back in 2010 during mass demonstrations, there was talk about how the military was trying to provoke violence, and use it as an excuse to take power.

The military, however, denied that, saying it is not going to take sides in the political dispute this time, our correspondent Wayne Hay reported.

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

Source: Agencies