Nepal's new prime minister K.P. Sharma Oli, tasked with rebuilding the quake-hit country and ending bitter constitutional protests, spent years in jail for trying to overthrow the king before becoming a tough-talking politician.

Oli, chief of the Communist Party of Nepal (Unified Marxist-Leninist) party, won a parliamentary vote to become prime minister on Sunday after last month's adoption of a new constitution.

The 63-year-old is charged with steering the recovery from a massive earthquake that killed nearly 8,900 people in April and left more than half a million homeless.

Oli also takes on the task of healing deep divisions over the new constitution.

It was meant to end centuries of inequality in the impoverished nation but has sparked deadly protests in Nepal's southern plains, where ethnic minorities oppose plans to divide the country into seven federal provinces.

Oli is known as a moderate within his party despite its communist leanings, and has vowed to work with other parties to develop the impoverished country.

He is credited with pushing the constitution through parliament, after eight years of cross-party negotiations made little progress. Lawmakers were spurred by the quake finally to shelve their differences.

But commentators say the veteran could struggle to appease the protesters, whose concerns he has publicly dismissed in the past.

"He has his work cut out for him because he has been the reason why this divisive constitution went through," said political commentator Kunda Dixit.

"You can't be a healer when you are the one who created the wound."

- Tough talker -

When he was 21, Oli was jailed for 14 years for leading a communist underground movement to overthrow King Mahendra's absolute rule.

After his release from prison, he married party worker Radhika Shakya and joined the Communist Party of Nepal (Unified Marxist-Leninist). Known as a tough talker, he rose through the ranks to become deputy leader of the opposition in parliament in 1999.

Oli lost the seat he had held since 1991 in parliamentary elections in 2008, when the former Maoist rebels entered politics and swept the polls with a wave of public support.

But the Maoists, who abolished the monarchy, struggled to hammer out an agreement on the constitution and Oli bounced back in 2013, when fresh elections saw his party come a close second to the Nepali Congress.

Commentators credit him with securing the cross-party deal in June on the long-delayed constitution.

But they warn he may struggle to contain the anger still simmering across Nepal's plains, where protesters have enforced a blockade at a key border checkpoint, cutting off vital fuel supplies to the landlocked country.

Oli, who secured his party's top job last year, openly scorned the protester's demands for a separate province and greater autonomy for their community.

"Identity is a treacherous wordplay. It is a cunning game to ignite social disharmony in Nepal," he said.

On Sunday he struck a more conciliatory note, telling lawmakers protesters' needs should be addressed.

Oli must also work to repair ties with giant influential neighbour India which supplies all of the impoverished country's fuel.

Oli accused India of deliberately blocking supplies to show its dissatisfaction with the constitution, a charge an angry New Delhi denied.

Oli has vowed to move quickly to rebuild the country after his predecessor came under fire for letting months pass after the quake with no progress on reconstruction.

Economist Bishamber Pyakurel said Oli's reputation as a decisive leader could be good news for Nepal's stuttering economy.

Following the quake it is forecast to grow around three percent this fiscal year, the lowest level in eight years.

"Out here, politics has always overshadowed economic concerns. We will have to see if Oli can take command and convince other parties... to prioritise issues of the economy and reconstruction," Pyakurel said.