Thailand's Prime Minister Prayuth Chan-ocha leaves after a press conference at the government house in Bangkok. Picture: Sakchai Lalit/AP

Bangkok - Thailand's general election "ran smoothly" but was still "deeply flawed," international observers said two days after the country's first election since a 2014 coup.

"The subsequent tabulation and consolidation of ballots were deeply flawed, which led to an announcement of some preliminary results that were wildly inaccurate on [the] election night," the Asian Network for Free Elections (ANFREL) said Tuesday in a statement.

"The lack of transparency and cooperation between the ECT [Election Commission] and other electoral stakeholders severely limited the outflow of information, thereby fueling further distrust in the process and the institution itself," it added.

The statement came amid widespread suspicion of irregularities in Sunday's election.

The Election Commission has delayed releasing preliminary results to Friday and will not announce the official results until early May. It has not addressed reports of miscalculated numbers and accusations of voter fraud.

The hashtag "Election Commission busted" has trended on social media since late Sunday.

With 94 per cent of the vote tallied, the pro-military Palang Pracharat Party was leading with 7.69 million votes, ahead of opposition Pheu Thai at 7.2 million and progressive Future Forward at 5.3 million.

But it was Pheu Thai that won the most constituency MPs at 137, while Palang Pracharat got 97 seats.

ANFREL has urged the commission to release comprehensive results as soon as possible to encourage and rebuild trust in the country's electoral system.

It remains unclear which parties will form the government.