Thailand's highly anticipated election results have been delayed as competing parties claim victory and concerns of irregularities and allegations of cheating grow.

The unofficial results from this national election ? the first democratic exercise since the 2014 military coup ? were originally expected on Sunday night, hours after polls closed.

While the Election Commission said some results would be ready in the coming hours, exact tallies were not due to be finalised until Friday. Even then, the party preferences would not have been calculated.

Preliminary, unofficial results for 350 parliamentary seats show that the Pheu Thai party leads with 138 seats, while the pro-military party Palang Pracharat holds 96.

The remaining 234 seats were shared among seven other parties.

There are an additional 150 "party list" seats, allocated under a complex formula, yet to be tallied.

The commission said full, official results would not be available until May 9.

The delays have caused anger and suspicion amongst some Thais about an Election Commission handpicked by the military junta.

This has been made worse by allegations of irregularities, with reports some polling stations had almost twice as many votes as registered voters.

Both sides claim victory

The commission, which said it had "nothing to hide", reported that the pro-military Palang Pracharat party was leading with 7.69 million votes and the the Pheu Thai party was trailing with 7.23 million votes.

The pro-army party, which will invite current Prime Minister Prayuth Chan-ocha to be an "outside Prime Minister" if successful, said it aimed to form a government after claiming it had won the popular vote, but the Pheu Thai party, which was ousted from government in the coup, said it won the most parliamentary seats and will try to form a coalition government with other parties.

The party, aligned with exiled former prime minister Thaksin Shinawatra, said it was also considering mounting a legal challenge over what it termed irregularities.

"There are irregularities in this election that we're not comfortable with. These affect the nation's credibility and people's trust," said Sudarat Keyuraphan, Pheu Thai Party's prime ministerial candidate.

"We will fight back through legal means."

Palang Pracharat party spokesman Kobsak Pootrakool said his party expected to gather 251 seats in the 500-seat House of Representatives ? a slim majority.

"Palang Pracharat will talk to like-minded parties who share the same ideology and standpoint to move the country forward, which will take a while," Mr Kobsak said.

The party wants to keep military junta chief Prayuth Chan-ocha in power.

A new, junta-designed electoral system could stack the deck in favour of the military party, critics alleged.

Petition to impeach

Meanwhile, amid popular dismay over the partial results, the number of signatures on an online petition to impeach the Election Commission leapt by more than 300,000 over a few hours to more than half a million.

A change.org petition launched a week ago to impeach the Election Commission garnered over 511,000 signatures by Monday, up from around 200,000 at the start of the day.

Many Thais took to social media to voice their suspicions about the results of an election that critics had said was systematically skewed in favour of the military from the outset.

Thai-language hashtags that translated as "Election Commission screw-up" and "cheating the election" were trending on Twitter in Thailand.

Many tweets referred to inconsistencies between the numbers for voter turnout and ballots cast in some parliamentary constituencies.

Some questioned the overall turnout of less than 70 per cent, which was much lower than expected.

Future Forward, a new party that appeared to have made a spectacular election debut thanks to its appeal to young voters, also questioned the poll numbers.

"There are obviously some irregularities with the numbers because they don't add up. This is making people sceptical of the election results," party spokeswoman Pannika Wanich said.

"The Election Commission should address this issue because if the people feel they cannot trust the results, there will be more problems to come."

ABC/wires