Thai oppn forms alliance, claims majority

Thailand's opposition Pheu Thai party claims its alliance has won enough lower house seats in the election to form a new government.

The opposition and ruling parties both claim victory in the Thai election. (AAP)

Thailand's Pheu Thai party says it has formed a "democratic front" with six other parties after a disputed election, claiming the opposition alliance had won a majority in the lower house and the right to try to form a government.

However, the coalition would likely fall short of electing a prime minister, which requires a combined vote with the upper house, which is entirely appointed by the military junta that in 2014 overthrew an elected Pheu Thai government.

The outcome of the election remains shrouded in doubt, with unofficial results delayed until at least Friday and allegations of vote-buying and irregularities in ballot counting.

Partial results still indicate the pro-army Palang Pracharat party will have enough votes to keep junta leader Prayuth Chan-ocha as prime minister. But an allied opposition majority in the lower house could lead to a political deadlock.

Pheu Thai's prime ministerial candidate Sudarat Keyuraphan told a news conference that together with the other parties, the opposition alliance would win at least 255 lower house seats, based on calculations drawn from partial results.

Pheu Thai secretary-general Phumtham Wechayachai told reporters the alliance includes Future Forward party, Pheu Chart, Prachachart, Seri Ruam Thai, Thai People Power and New Economy.

The pro-military Palang Pracharat party, which won 97 lower house seats, also claimed the right to form the next government due to its lead on Pheu Thai in the popular vote.

A group of international observers has criticised vote counting , saying the "tabulation and consolidation of ballots were deeply flawed" though it had no reason to believe the issues affected overall results.

The Asian Network for Free Elections on Tuesday said the announcement of some preliminary results that were "wildly inaccurate" damaged the "perceived integrity of the general election."

The group, also known by its acronym Anfrel, is one of several observer groups that have raised concerns about Sunday's vote.

Thailand's Election Commission, appointed by the junta's hand-picked legislature, blamed the media for failing to keep up.

The commission said it will release its final preliminary results on Friday. Official results are not expected until May.