Thai election officials have warned that it could take several weeks to re-stage voting in areas where Sunday's election was disrupted by protesters.
Anti-government protesters halted voting in parts of Bangkok and the south by blockading polling stations, rendering millions unable to vote.
Former PM and opposition leader Abhisit Vejjajiva told the BBC the government had "lost the trust of the people".
He said people had no faith in the system and that reform was essential.
"We are saying the government needs to recognise it has lost trust of people because of its abuse of democratic processes," Mr Abhisit told the BBC's Jonathan Head in Bangkok.
The ruling party of Yingluck Shinawatra is expected to win the election but legal challenges and a lack of MPs as a result of the disruption to voting may create political limbo.
The protesters, many of whom are still out on the streets, allege that Ms Yingluck's government is controlled by her brother, ousted leader Thaksin Shinawatra. They accuse Thaksin-allied parties of buying rural votes with ill-judged schemes that hurt the economy.
Mr Abhisit said all sides accepted that elections were necessary, but disagreements were about the measures put in place "so people have trust that these elections are free and fair and that they will actually lead to an achievement of reforms".
His Democrat Party, which is the main opposition and allied to the protesters, boycotted the elections on Sunday. It has been unable to win a majority in parliament for more than two decades.
He denied that responsibility for the failure to reach consensus also lay with the opposition bloc: "What we have certainly done is offered alternatives and a way out which the government has refused to accept," he insisted.
'Right to democracy'
The disruption means not all seats in parliament will be filled, requiring by-elections in many places.
The government wants elections that were disrupted to be re-run as soon as possible - a new parliament cannot sit until 95% of seats have been filled, reports our correspondent.
But the official election commission has warned it may take weeks to hold by-elections in so many constituencies.
Prime Minister Yingluck Shinawatra called the snap election after a sustained campaign by the protesters. She said on Sunday that going to the polls had been the right thing to do.
"At least I think at this election it is very important that people come out to vote for their right to democracy," she said.
Protest leader Suthep Thaugsuban, however, told supporters late on Sunday he was "confident this election won't lead to the formation of a new government".
Thailand's Election Commission said six million voters were affected by disruption on Sunday, but 89% of polling stations operated normally.
On Monday protesters again took to the streets, marching through parts of Bangkok.
"We are not giving up the fight. We still keep fighting,'' Mr Suthep said. "Our mission is to keep shutting down government offices, so don't ask us to give those back.''
The protesters are to reduce the number of major road junctions they have blockaded in Bangkok from seven to five. They cited fear of attack as the reason, though some observers pointed to dwindling numbers.
The protesters want the government to be replaced with an unelected "people's council" to reform the political system.
They allege that Ms Yingluck's government is controlled by her brother, ousted leader Thaksin Shinawatra. They accuse Thaksin-allied parties of buying rural votes with ill-judged schemes that hurt the economy.
The opposition says it will challenge the poll as it "did not reflect the intention of the constitution or the people".
Ms Yingluck's party is already facing a host of challenges in the courts that could force it from power, as has happened with pro-Thaksin parties in the past.