By Chayut Setboonsarng and Panarat Thepgumpanat

BANGKOK (Reuters) -The leader of the progressive Move Forward Party that delivered a surprise victory in Thailand's election said on Thursday he was confident he could muster additional support from rivals to back his alliance and form a stable government.

Move Forward, which won most seats after it capitalised on its groundswell of youth support, has an alliance of eight parties worth about 313 of the 500 lower house seats but no guarantees it can form a coalition government.

It may need to lobby some of its rivals and charm detractors in a conservative-leaning Senate with which the party has previously clashed over its liberal agenda.

"There is a committee and negotiation team in place to find out what I further need, the seats I need, so there is stability and no loss of balance in governing," Move Forward leader Pita Limjaroenrat told a press conference.

He added: "My coalition is taking shape. And we have a very clear roadmap from today and until the day I become PM."

Pita and his partners say they have a mandate from the electorate to end nearly a decade of conservative, army-backed rule in Southeast Asia's second-largest economy.

The alliance overnight added two more members and three seats but it still appears short of the 376 votes needed from the 750-member bicameral legislature to vote in a prime minister to form a government.

Pita's big challenge is winning votes from the 250 members of the upper house Senate, a chamber that was appointed by a junta after a 2014 coup and has a record of siding with army-backed parties.

MISSION AHEAD

Those parties were thrashed in Sunday's election by Move Forward and the populist heavyweight Pheu Thai, but the prospect of a pro-military bloc forming a minority government - assuming they have the Senate's support - cannot be ruled out.

Move Forward won massive youth support with a lively campaign and sophisticated use of social media, but its anti-establishment stance on some issues, including over business monopolies, could complicate its bid to rule.

The U.S.-educated Pita, 42, was dealt a blow late on Wednesday when the third-place finisher Bhumjaithai - a potential game-changer with its 70 seats - said it could not back any prime minister who supports amending or abolishing a law against insulting the powerful monarchy.

Move Forward campaigned on changing the lese-majeste law, under which at least 200 people have been charged in the past few years, many from a youth-led protest movement. The law prescribes jail terms of up to 15 years for each perceived offence, with some given sentences of several decades.

Asked about Bhumjaithai's declaration, Pita said: "That is their matter. The eight parties have a position and clarity."

It is far from certain that the new alliance would become Thailand's next government, despite a decisive opposition victory.

Move Forward in the coming days aims to thrash out an agreement with the other seven parties that will include finding common ground on some complex issues. Arguably the most thorny is the taboo issue of lese-majeste.

Pita also brushed off a pending case filed with the election commission seeking to disqualify him over shares he allegedly holds in a media company, which could be a violation of rules.

"I'm not worried ... I understand there are many dimensions in politics," he said. "As a public figure I can accept the investigation."

(Reporting by Chayut Setboonsarng and Panarat Thepgumpanat; Additional reporting by Devjyot Ghoshal; Writing by Martin Petty; Editing by Robert Birsel)