Opposition parties are leading in Sunday's Thai general election, according to preliminary results released by the electoral body.

The two major opposition parties, the Move Forward Party and the Pheu Thai Party, were almost on par in the race for the 400 constituency seats in the House of Representatives or the lower house of parliament where they were forecasted to capture half of the seats.

According to a live update by the country's Election Commission, nearly 80 percent of the votes were counted by midnight.

The Move Forward Party, with wide support among young voters, emerged strong in the election and was leading in the separate poll to decide the 100 party-list seats in the House with more than 9.9 million votes, ahead of Pheu Thai's 7.8 million votes.

Move Forward leader Pita Limjaroenrat, a 42-year-old former executive of a ride-hailing app, described the outcome as "sensational" and vowed to stay true to his party's values when forming a government.

He said he remained open to an alliance with Pheu Thai, but has set his sights set on being prime minister.

"It is now clear the Move Forward Party has received the overwhelming support from the people around the country," he said on Twitter.

Pheu Thai leader Paetongtarn Shinawatra, 36, said she was happy for Move Forward, but it was too soon to discuss alliances. "The voice of the people is most important."

Pheu Thai, founded by polarizing self-exiled tycoon Thaksin Shinawatra, remains hugely popular among the working classes and was banking on being swept back to power in a landslide on nostalgia for its populist policies like cheap healthcare, micro-loans and generous farming subsidies.

Prime Minister Prayut Chan-o-cha's Ruam Thai Sang Chart Party (United Thai Nation Party) is projected to take some 24 constituency seats but was ranked third in the party list poll with more than 3.2 million votes.

Prayut had campaigned on continuity after nine years in charge, warning a change in government could lead to conflict.

On Sunday, he slipped away quietly from his United Thai Nation party headquarters, where there were few supporters to be seen.

"I hope the country will be peaceful and prosper," Prayut told reporters.

More than 52 million Thai citizens were eligible to vote in the election. Under the two-ballot system with one vote for the 400 constituency members of parliament (MPs) and another for their preferred party vying for the 100 party-list MPs, voters will elect a total of 500 members to the House of Representatives.

The Election Commission is yet to announce the preliminary allocation of the 100 party-list seats. After the tally is finished, the official results are pending verification by the Election Commission, which is expected to be completed within 60 days.

The new MPs will then join the 250 senators to elect a new prime minister with a simple majority in accordance with the 2017 constitution.

(With input from agencies)