Red-shirt supporters and all others who love democracy should turn out en masse to vote on Sunday, says Tida Tawornseth, chairwoman of the United Front for Democracy against Dictatorship (UDD).
She issued the call as anti-government protest leader Suthep Thuagsuban threatened to completely shut down Bangkok to prevent voting from taking place.
Mr Suthep had earlier pledged that his People's Democratic Reform Committee (PDRC) would not actively impede people from voting. But that pledge was broken in many places last weekend and a lot of polling places are expected to be blocked on Sunday.
Mrs Tida said on Saturday that all eligible voters should turn out to exercise their constitutional rights to show that they demand a democratic system.
She said she expected election officials might not show up at some polling stations and urged people in the areas, particularly red-shirts, to volunteer to replace them to enable the polls to go ahead.
The UDD chief also called on people to closely follow up the elections in their constituencies, including the delivery of ballot boxes and voting papers for storage or counting.
If people find any electoral law violations, they should take photographs for use as evidence in taking legal action against wrongdoers. They can submit the pictures to police, political parties or UDD co-leaders, she said.
Mrs Tida also warned the election officials to put their utmost effort into performing their duties, or face charges of negligence under the Article 157 of the Criminal Code.
Election Commission officials have been reluctant to go ahead with Sunday's poll because of concerns about safety. However, caretaker Prime Minister Yingluck Shinawatra insisted that voting must take place.
The election, which is being boycotted by the opposition Democrat Party, is costing taxpayers 3.8 billion baht (US$115 million) but will not produce enough MPs to convene a sitting of the House of Representatives. That could take another four or five months of byelections, barring legal challenges, and add to the costs.