By Panarat Thepgumpanat and Chayut Setboonsarng

BANGKOK Voting was underway in Thailand's parliament for a new prime minister on Thursday, with Move Forward Party leader Pita Limjaroenrat running unopposed in what will be a critical test of his political clout.

The 42-year-old, U.S.-educated liberal faces a big challenge in securing the required backing of more than half of the 749-member bicameral parliament, where many lawmakers are opposed to his party's anti-establishment agenda, including a controversial plan to amend a law that prohibits insults of the monarchy.

The progressive Move Forward and its alliance partner, Pheu Thai, thrashed conservative pro-military parties in the May 14 election, seen widely as a resounding rejection of nearly a decade of government led or backed by the royalist military.

But Pita's determination to pursue Move Forward's agenda puts him at odds with a powerful nexus of conservatives and old-money families that have loomed large over Thai politics for decades, and will be almost certain to try to thwart him on Thursday.

His alliance controls 312 seats, but to get the required 375 votes he needs support from some of the 249 members of the conservative-leaning upper house Senate, which was appointed by the military after a 2014 coup. There were 676 lawmakers in attendance for the vote.

"I will do my best to match the hope and encouragement from the people," Pita told reporters as he arrived at parliament.

"I'll use this opportunity to find a consensus."

(Reporting by Panarat Thepgumpanat, Panu Wongcha-um, Orathai Sriring and Chayut Setboonsarng; Writing by Martin Petty; Editing by Lincoln Feast and Kanupriya Kapoor)