Late on Monday afternoon, the Election Commission (EC) said with 95% of the vote counted, its latest unofficial figures gave Pheu Thai 137 constituency seats and Palang Pracharat 79, followed by Bhumjaithai with 39, the Democrats with 33 and Future Forward with 30. 150 seats were yet to be decided.
Earlier, Secretary-general Ittiporn Boonprakong said the EC would announce the unofficial results of 350 constituency seats later on Monday, but unofficial full vote counts -- which are needed to determine the allocation of 150 other seats in parliament -- would not be available until Friday. He reiterated that official results from at least 95% of House seats would not be finalised until May 9.
Khunying Sudarat Keyuraphan
The Pheu Thai party, which was ousted from government in the coup, said it won the most constituency seats in Sunday's election and will try to form a government with similar-minded parties. But the unofficial results released thus far show the military-backed Palang Pracharat party won the popular vote.
On Sunday night, the EC delayed without explanation a full announcement of preliminary results as a blizzard of complaints mounted over apparent mistakes in the count and possible irregularities at the polls.
Nearly 1.9 million votes had been invalidated with 93% of votes tallied, the EC said late Sunday. Earlier counts showed that in a handful of provinces more than half the ballots cast were invalidated.
Sunday's election -- seen as a referendum on the military -- was held under new rules written by the junta to ease its transformation into a civilian government.
Despite that headstart, analysts had not expected the army-linked Palang Pracharat party to win the popular vote, given anger at junta rule and the enduring popularity of Pheu Thai -- the party of ex-premier Thaksin Shinawatra.
But as initial figures dribbled out, Palang Pracharat -- with 2014 coup leader Prayut Chan-o-Cha as its candidate for prime minister -- edged into a clear lead, racking up more than 7.6 million votes with more than 90% of ballots tallied.
"This will give them... popular legitimacy and more credibility," Thitinan Pongsudhirak, a political scientist at Bangkok's Chulalongkorn University, told AFP.
Palang Pracharat had nearly half a million more votes than Pheu Thai, despite the track record of Thaksin whose parties have won every vote since 2001, drawing on loyalty from the rural and urban poor.
That should propel Gen Prayut's party to the 126 lower house seats it needs to secure a parliamentary majority, in combination with a 250-seat upper house Senate that is appointed by the junta.
Shock gripped the pro-democracy camp, which had hoped to harness widespread anti-junta sentiment.
Gen Prayut Chan-o-cha
Thaksin, the self-exiled billionaire ex-cop turned-Thai premier, who is at the crux of Thailand's political breakdown, reached out to supporters as the results tipped against his Pheu Thai party.
"As long as we still have breath, we can not give up," he said in a Facebook post on Monday.
Prospects of an anti-junta coalition squeezing into power began to evaporate late Sunday.
The Election Commission has said it will finalise the results by May 9.
But questions over the count have billowed out, with social media ablaze with allegations of vote buying, mass invalidation of electoral cards and bungling by polling staff across the country.
About 400,000 people signed a change.org petition to dissolve the commission.
Three candidates from the junta's Palang Pracharat were the first to file petitions for a recount after losing in their constituencies.
More disputes are expected across the political spectrum.
Turnout was also 64%, much lower than forecasts.
A coordinator for Thailand-based election monitor We Watch said that voter education was "really insufficient" for the first election in eight years.
Voters were, for example, confused about "how to make the ballot valid, how to mark the ballot," We Watch's Chompunut Chalieobun told AFP.
Mr Chompunut also said many monitors were blocked from accessing polling stations.
Election night hoisted a new, youth-focused political party with vigorous anti-junta positions -- Future Forward party -- onto the Thai political stage.
Led by telegenic billionaire Thanathorn Juangroongruangkit, Future Forward took more than five million votes, which could make it Thailand's third-biggest party.
And on the eve of the poll, His Majesty King Maha Vajiralongkorn issued a cryptic statement urging people to support "good" leaders against those who create "chaos".
Another royal command in February torpedoed the candidacy of the king's elder sister Princess Ubolratana for prime minister via a party linked to Thaksin.
Thaksin was ousted in a 2006 coup, and has lived in self-exile since 2008. The administration of his sister Yingluck was also the victim of a 2014 power grab by the military.
The Shinawatra-aligned Pheu Thai party has vowed to accept the results but said it needed more "clarity" on the outcome before it took a stance.