Pheu Thai party's candidate for prime minister Sudarat Keyuraphan (R) leaves after speaking at a press conference in Bangkok on March 25. (Photo: VCG)

Seven Thai political parties led by one ousted from power in a military coup said Wednesday that they believed they won enough seats in the recent general election to form the next government.

The announcement came amid concerns about potential irregularities in Sunday's vote, the first since a 2014 coup, and before a full preliminary vote count has even been released. Official results aren't due until May.

The leader of the Pheu Thai party, which headed the last elected government, and the leaders of five other parties held a news conference to say they believed they had won more than 250 seats in the 500-seat lower house. They said another smaller had also agreed to join their potential government.

"Today, we hold the fact that parties on the side of democracy have received majority support from the people, although figures are not stable yet," Pheu Thai's Sudarat Keyuraphan said.

The Election Commission, appointed by the ruling junta's hand-picked legislature, delayed the release of a full preliminary vote count on election night and then again on Monday. The commission has now said it will release its final preliminary results on Friday.

The incomplete results reported so far show Pheu Thai won the most seats, while the military-backed Palang Pracharath party appeared to have received the most votes. Both have said those results mean they have a mandate to form the next government.

Even if the Pheu Thai-led coalition comes together and it secures more than 250 lower house seats, it may not get to form the government due to the country's new political system.

Since the coup, the junta has used the absolute power it granted itself to go about rewriting the country's laws, including commissioning a new constitution and creating an electoral system that severely handicaps parties without links to the military.

Under the new rules, an unelected 250-member Senate appointed by the junta will join the 500 elected members of the lower house in a vote for prime minister, meaning a party without Senate support would have to get the support of 376 house members to ensure its choice becomes premier.

The vote for prime minister will likely take place sometime in May and give Palang Pracharath's candidate, current junta leader and Prime Minister Prayuth Chan-ocha, a considerable advantage.