Thailand's Move Forward party announced on Monday that it had sufficient votes to form a coalition government but a military-appointed Senate, the party's position on a royal insult law and a complaint against its leader may stand in the way.

Pita Limjareonrat, 42, led the Move Forward party to a stunning victory in Sunday's general election, winning the highest number of seats, ahead of another opposition party, the political heavyweight Pheu Thai.

The victory of the two opposition parties may pave the way to ending nearly 10 years of military-backed governments led by a former army chief, Prime Minister Prayuth Chan-ocha, whose newly formed party won a small fraction of the seats that the opposition parties did.

"I am ready to become Thailand's 30th prime minister," Pita declared, explaining that his party and its five prospective coalition partners, including Pheu Thai, would secure 309 seats in the 500-seat lower house of parliament.

However, to be indisputably in a position to become prime minister he needs to be able to command a majority in a joint sitting of the bicameral legislature, which includes 250 members of a military-appointed Senate.

So he needs 376 members of a joint session to vote for him.

Reuters spoke to six senators to try to gauge the mood of the upper house. Some of them suggested they would not necessarily vote with the majority in parliament, even though that reflected the will of the people as expressed on Sunday.

Senator Somchai Sawangkarn said his vote for who becomes prime minister would based on his criteria and a lower-house majority alone was not sufficient.

"The person must be honest and not cause problems in the country," Somchai said.

"Hitler was elected in a majority but led the country to world war ... If there is a possibility of creating division in the country, I will not vote for them," he said.

Another, Kittisak Rattanawaraha, said the next leader must be loyal to the nation, religion and king and not corrupt, echoing themes upheld by Move Forward's conservative opponents.

A polarising issue for Move Forward is its position on amending a strict royal insult law, which sets out a sentence of up to 15 years for defaming the monarchy.

Critics says conservative governments have used the law to stifle dissent but conservatives are fiercely opposed to any suggestion of amending it.

The royal family is officially above politics and the king constitutionally enshrined to be held in "revered worship".

Senator Jet Sirathananon said he would respect the wishes of the majority.

"The Senate should not block the work of parliament. Based on what we saw yesterday, we'll respect people's votes," he said.

One senator said he would abstain on the grounds that it was the duty of the lower house to select the prime minister.

Another danger that Pita faces could come from the courts.

According to a complaint filed with the Election Commission before the vote, Pita broke electoral rules because he holds shares in a media company.

Pita said he was ready to explain that there was no wrongdoing and the allegation was a distraction.

"The road for Move Forward is just starting and it will not be smooth," said Ben Kiatkwankul, partner at Maverick Consulting Group, government affairs advisory. (Additional reporting by Chayut Setboonsarng; Editing by Robert Birsel)