Thailand's Prime Minister Prayut Chan-o-cha has called for unity after the military-backed ruling parties suffered a heavy defeat in the general election.

Prayut's United Thai Nation Party only secured 36 seats in the 500-member Lower House.

On Tuesday, he congratulated the opposition parties for their victory. He said he will continue to perform his duties until the formation of the new government.

He also called on people to work together to develop the country.

"I ask every fellow Thai citizen to join hands in building love and unity as well as stability in the country without any conflict", he said.

But he declined to answer a question about whether he will remain active in politics.

The Move Forward Party which won highest number of seats with 152, held a victory parade in Bangkok late on Monday.

The party is led by Pita Limjaroenrat, a 42-year-old Harvard graduate. It campaigned on progressive ideas and criticized the military's influence in Thai politics.

The focus is on whether Move Forward is able to form a coalition with the largest opposition party, Pheu Thai. It is affiliated with ousted former premier Thaksin Shinawatra and placed second with 141 seats.

It is still possible that pro-military parties could choose the next prime minister with support of the Senate whose members were effectively picked by the military.