The leader of Thailand's opposition Move Forward Party said on Monday he had been in contact with at least five opposition parties on forming a coalition and warned any attempt to interfere in the election outcome will come at "a hefty price".

Pita Limjaroenrat (above), 42, whose party came in first in Sunday's election, closely followed by populist opposition heavyweight Pheu Thai, said he has proposed an alliance that would command 309 seats and he was ready to be prime minister.

He said all sides must respect the election outcome and there was no use going against it.

"I am not worried but I am not careless," he told a press conference.

"It will be quite a hefty price to pay if someone is thinking about debunking the election result or forming a minority government."

Between them Move Forward and Pheu Thai trounced parties with ties to the royalist army on Sunday, but it is far from certain the opposition will form the next government, with parliamentary rules drafted by the military after a 2014 coup skewed in favour of its allies.

To govern, agreements may need to be struck with multiple camps, including rival parties and members of a junta-appointed Senate with a record of favouring conservative parties led by generals.

The Senate takes part in a combined vote of the 750-seat bicameral parliament on who becomes prime minister and forms the government. The support of more than half of the two houses, or 376 votes, is needed.

Pheu Thai has yet to announce any alliance plans. Pita said there was positive feedback from the other parties to his proposal.

Military parties down, but not out

Though the preliminary election results appear to be a hammer blow for the military and its allies, with parliamentary rules on their side and some influential power-brokers behind them, they could still have a role in government.

Move Forward was galvanised by a wave of excitement among the youth over its liberal agenda and promises of bold changes, including breaking up monopolies and reforming a law on insulting the monarchy.

The party made inroads in some conservative strongholds and added a new dimension to the battle for power that was for years centred on the billionaire Shinawatra family, the driving force behind Pheu Thai, and a pro-military establishment, that brought two decades of on-off tumult.

Pita said Move Forward would press ahead with its plan to amend strict lese majeste laws against insulting the monarchy, which critics say have been used to stifle free speech. Thailand's palace does not comment on the law or its use.

The law punishes perceived insults by up to 15 years in prison, with hundreds of people facing charges, some of whom are in pre-trial detention.

Pita said parliament would be the right forum to seek amendments to the law.

"We will use the parliament to make sure that there is a comprehensive discussion with maturity, with transparency in how we should move forward in terms of the relationship between the monarchy and the masses," he said.

- Reuters