The Democrats have not won an election since 1992, and some of their leaders appear to have given up on electoral politics because they cannot win. The protesters are demanding a non-elected people's council lead the country instead.

Yingluck said again yesterday that she was not trying to cling to power and would be "happy to resign" and dissolve parliament if that could ease the crisis. But she said those things could only happen if new elections were organised within 60 days and all parties accepted the outcome.

Protest leader Suthep Thaugsuban has repeatedly rejected those initiatives and refused to negotiate.

Yingluck also reiterated an offer to set up a national forum to find a way out of the crisis. She said if there was still no resolution, a national referendum could be held, but she did not specify on what.

Any "government that comes to power without elections would significantly affect our image and confidence in the country," Yingluck said.

Thailand has been plagued by political turmoil since Yingluck's brother Thaksin Shinawatra, a former premier, was toppled in a military coup in 2006.

At least five people have been killed and 289 injured since the latest unrest began. Protesters have vowed a final showdown today in Bangkok and will march en masse from a government complex they seized to Yingluck's office.

The Democrats held 153 of the 500 seats in the lower house.

Democrat leader and former premier Abhisit Vejjajiva said the resignations were immediate.

He said Yingluck's government had 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."