Nepal's new Prime Minister Pushpa Kamal Dahal pledged Wednesday to unite a country hit by deadly protests over constitutional changes after lawmakers elected the Maoist leader and former guerrilla to head a new government.

Dahal, better known by his nom de guerre Prachanda or "the fierce one", led a decade-long Maoist insurgency before transforming the rebel movement into a political party after a 2006 peace deal.

He is now tasked with rebuilding the country after a devastating earthquake and resolving simmering unrest over the new national charter adopted last September.

"I pledge that I will unite everyone in the country," he said in an address to parliament before the vote.

"I have a responsibility to solve this problem. I feel that I have been put forward as a candidate to work as a bridge between (communities)," he said.

Dahal will also have to mend relations with powerful neighbour India, which made no secret of its disapproval of the constitution.

Indians living near the Nepali border have close links with Nepal's Madhesi ethnic minority, which complains that the new charter leaves them marginalised.

Kathmandu in turn accused New Delhi of supporting a months-long border blockade by Madhesi protesters that cut off vital supplies from India and prompted fuel-starved Nepal to turn to China for help.

Following Wednesday's election, Indian Prime Minister Narendra Modi tweeted that New Delhi would offer Dahal "our full support" and invited him to visit the country -- a traditional first stop for Nepali prime ministers heading overseas.

Commentators cautioned that the brash leader, who has vowed to honour the previous government's economic agreements with China, will need to tread carefully to win New Delhi's confidence without losing Beijing's support.

"He should balance relations with both neighbours," said Guna Raj Luitel, editor of Nepali daily Nagarik.

"They are both economic powers and he should try to win benefits for Nepal from both," Luitel told AFP.

Dahal, head of the Maoist party, ran unopposed and secured 363 out of 573 votes to become the Himalayan nation's premier for a second time.

He has served as prime minister once before, after the Maoists won elections in 2008, but only lasted nine months in office before resigning.

Dahal, whose Maoists are the third-largest force in parliament, pulled out of a coalition led by Khadga Prasad Sharma Oli -- the former prime minister -- three weeks ago and secured the support of the biggest party, the Nepali Congress.

He also won the backing of the United Democratic Madhesi Front (UDMF) -- an alliance of protesting parties -- paving the way for warmer ties between the new government and Madhesi demonstrators.

- 'Power games' -

Oli, who heads the Communist Party of Nepal (Unified Marxist Leninist), faced fierce criticism over his handling of the protests.

More than 50 people died in clashes between police and protesters demanding revisions to federal state borders set out in the new charter.

Dahal is expected to be more sympathetic to protesters' concerns following the Madhesi parties' decision to back his candidacy.

But any changes to the constitution must be passed in parliament by a two-thirds majority, meaning he will likely need to reach out to Oli's UML, which holds 182 out of 595 seats, for support.

The new charter, the first drawn up by elected representatives, was meant to bolster Nepal's transformation from a Hindu monarchy to a democratic republic after decades of political instability.

But continuing discussions between the government and protesters have failed to break the deadlock.

Meanwhile, some four million Nepalis are still living in tents or huts after an earthquake that killed nearly 9,000 people in 2015.

Nepalis frustrated by the slow pace of reconstruction said they had few hopes for the new government.

"So many who lost their homes in the earthquake are still in shelters. But (politicians) are always busy playing power games," said Nabin Shrestha, who drives a three-wheeler for a living.

Prachanda will be the eighth prime minister to lead Nepal since 2008, when he became premier for the first time and overthrew a 240-year-old feudal monarchy.