Thai Prime Minister Thaksin Shinawatra has claimed re-election victory in Sunday's poll after exit polls predicted a massive win for his party.
Thaksin said a single-party cabinet would speed things up
Exit polls gave the Thai Rak Thai (Thais Love Thais) party as many as 399 seats in the 500-seat parliament.
"The numbers are more than enough to establish a one-party government," Mr Thaksin declared.
The main opposition Democrat party has admitted defeat, saying Thailand could become a "parliamentary dictatorship".
500 posts in House of Representatives
400 chosen from constituencies and 100 from party lists
Economy, southern violence and tsunami relief seen as key issues
If the landslide victory is confirmed, it will be the first single-party cabinet under Thai democratic rule.
Mr Thaksin's handling of the 26 December tsunami, and the Thai economy's strong performance, appear to have convinced voters to give him an unprecedented second consecutive term, observers say.
Policies such as providing health care for less than a dollar, and promises to eradicate poverty across the kingdom, have made the telecoms billionaire immensely popular among the poor.
Even more controversial measures, such as the war on drugs in which more than 2,000 people have been killed - have had overwhelming popular approval.
The BBC's Kylie Morris says the result is a devastating blow for the opposition, who had hoped to win at least 201 seats to be able to censure the government in parliament.
"I was shocked when I saw the exit polls, that Thai Rak Thai managed to win 399 (seats) and the Democrats won only 80," Democrat party chief Banyat Bantadtan said.
Mr Banyat also alleged that vote-buying had increased on the eve of the poll.
"There was a huge vote-buying effort last night," in southern provinces, which are seen as Democrat strongholds, Mr Banyat told Thai television.
The electoral commission banned voters from taking mobile phones into polling booths in an attempt to stop vote-buying.
It suspected that people would use phones with cameras to photograph their ballot papers, to prove that they voted for the party offering to buy their vote.
Mr Thaksin has rebuffed suggestions that a landslide win would render his government autocratic.
"The constitution doesn't say how many seats must be reserved for the opposition, so I don't understand why the Democrats claim that our popularity makes Thai Rak Thai a parliamentary dictatorship," he said.
Official results are not expected to be announced until at least Wednesday.