Thai voters have rejected the military-backed administration, and two opposition parties look to be on the verge of forming a coalition.
According to preliminary findings, the Move Forward and Pheu Thai parties have surged ahead of Prime Minister Prayuth Chan-ocha.
The election has been hailed as a watershed moment for Thailand, which has recently suffered military coups.
Mr Prayuth led the previous coup in 2014 and sought re-election.
However, he has faced stiff election challenges from the anti-military groups Move Forward and Pheu Thai.
Move Forward is managed by former IT CEO Pita Limjaroenrat, whereas Pheu Thai is led by Paetongtarn Shinawatra, the daughter of former Prime Minister Thaksin Shinawatra.
Vote counting began on Sunday, nine hours after polls closed at 17:00 (10:00 GMT) at 95,000 polling sites around Thailand.
Around 50 million people were expected to vote to elect 500 members of the lower house of parliament, with two million voting early.
With 97% of the votes counted as of 03:30 (21:30 GMT), Reuters predicted that Move Forward would win the most seats, with Pheu Thai coming in second.
The Thai election is being shaken up by young radicals.