BANGKOK (AFP) - Thailand's main opposition party announced on Sunday that all of its lawmakers would resign en masse, deepening the kingdom's political crisis as anti-government protesters prepare for another major rally.

Democrat Party spokesman Chavanond Intarakom told AFP the Members of Parliament would formally step down "as soon as possible".

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", with protesters vowing a final showdown on Monday.

Protesters have vowed a final showdown on Monday in efforts to topple Ms 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 lawmaker Sirichok Sopha said in televised remarks.

Ms 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.

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.

Nine opposition MPs resigned earlier this year to lead the mass protests.