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 MPs 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.