Parliament's upper house, entirely appointed by the junta, is expected to endorse the pro-military party
Thailand's opposition "democratic front" of seven parties on Wednesday said it had won a majority in the lower house of parliament after a messy election, and had the right to try to form a government after five years of military rule.
But the opposition alliance would still be unable to elect a prime minister, as parliamentary rules, written by the ruling military junta, require backing from a majority of upper and lower houses combined.
Parliament's upper house, entirely appointed by the junta, is expected to endorse the pro-military party.
With unofficial results of Sunday's vote still delayed, the ruling junta showed no sign giving up on its goal to keep former army chief and coup leader Prayuth Chan-ocha as prime minister.
The post-election standoff could raise tension just as the Southeast Asian country prepares for the elaborate coronation of its new king in May.
Sudarat Keyuraphan, the main prime ministerial candidate of the Pheu Thai party ousted by the 2014 army coup, said the seven parties in the opposition alliance would take at least 255 seats in the 500-member House of Representatives.
"We declare that the democratic front which opposes military rule commands the majority in the House," Sudarat told reporters.
The largest alliance in the lower house should be given the right to try to form a government, she added.
"Parties in the democratic front gained the most trust from the people," Sudarat said, adding that the alliance would start courting more parties on Wednesday.
Pheu Thai's secretary-general, Phumtham Wechayachai, told reporters the democratic front now included the Future Forward, Pheu Chart, Prachachart, Seri Ruam Thai, Thai People Power and New Economy parties.
Missing was the Bhumjaithai Party, another key vote getter that has not yet aligned with either side.
Responding to speculation that Pheu Thai would offer the top post to Bhumjaithai's leader, Anutin Charnvirakul, to win him over, Phumtham said, "We haven't made him that offer."
He added, "Premiership is something we will discuss later. It's not a pre-condition for us."
A fuller picture of the lower house make-up could emerge on Friday, when the election commission releases vote tallies for each constituency, used to determine the allocation of the other 150 party seats under a complex formula.
After the news conference, the alliance's party leaders petitioned the election commission to immediately release complete tallies, demanding transparency.
Ittiporn Boonprakong, the commission's chairman, said the votes were all counted in the open and announced at polling stations the night of the election.
'Still time yet'
The pro-military Palang Pracharat party has also claimed the right to form the next government, based on its early lead in the popular vote, saying the count could still give it a majority.
Its leader, Uttama Savanayana said Palang Pracharat was taking its time as there are six weeks until official results are published on May 9.
"We've started talking to other parties, but there's still time yet," Uttama told reporters. "We'll try to get as many seats as possible. We're confident we can form a government."
Party secretary-general Sontirat Sontijirawong dismissed the claim by the democratic front.
"Stop claiming to be on the side of democracy," Sontirat said. "This election was democratic. Are the 7.9 million who voted for Palang Pracharat not democratic?"
The partial count suggests Palang Pracharat could win enough elected lower house seats, combined with votes from the junta-appointed Senate, for Prayuth to stay on as prime minister.
However, Prayuth could face parliamentary deadlock if the opposition controls the lower house, and would be vulnerable to a confidence vote.