Lead campaigners include the Thammasat University Student Union, advocacy groups for women and a number of student chapters.

"Voters will be encouraged to go to the balloting stations and cast a 'no-vote' ballot in a gesture of protest against the election, which is deemed undemocratic," Thammasat student leader Salinee Ratanachaisit said.

Activist Ticha na Nakhon called on women to deny Prime Minister Thaksin Shinawatra a chance to return by marking a "no-vote".

Thaksin has refused to resign and Sunday's election appears set to go ahead, but voters can opt for the "no-vote" box to prevent him from using the outcome of the election to cling to power, Ticha said.

 The embattled prime minister has pledged not to lead the next government if his Thai Rak Thai Party receives less than 50 per cent of votes cast.