A crude bomb exploded early on election day in Nepal, injuring three children, as voters cast their ballots in a poll seen as crucial in completing the nation's stalled peace process.

The bomb, planted in a residential suburb in the capital Kathmandu detonated three hours after polling stations opened.

Police say an eight-year-old picked up the bomb, assuming it was a toy, before it exploded.

A splinter group of the Maoist party has vowed to disrupt the poll.

The general election is only the second since the 10-year civil war launched by Maoist rebels concluded in 2006, ending royal rule and transforming Nepal into a secular republic.

On the eve of the polls, Nepal's chief election commissioner Neel Kantha Uprety said the country would witness a "free, fair and fearless" vote despite recent violence from anti-election hardliners.

"The security situation is managed in such a way that all voters will be safe not only in the polling booths but also on the way there and back home," Mr Uprety added.

Nepalis flocked to the ballot box in the first constituent assembly elections in 2008 and delivered an overwhelming victory to the Maoist party, but have since grown frustrated following years of political infighting.

A series of coalition governments have failed to draft a constitution and complete the peace process, leading to the collapse of the first constituent assembly in May 2012.

The Maoists, led by former rebel leader Pushpa Kamal Dahal, known better as Prachanda, have since split, with a hardline faction boycotting Tuesday's polls.

Demonstrators from the splinter group have escalated efforts to disrupt the vote in recent days, torching vehicles and hurling explosives at traffic. Police have arrested more than 310 people.

Dev Gurung, a senior leader in the splinter Communist Party of Nepal-Maoist (CPN-M), told AFP he hoped voters would stay home.

"It's within our right to boycott the polls," he said.

"There is no use for this election. It will not be able to write a people's constitution."

50,000 police deployed at polling stations

More than 100 parties, including three major ones - the Unified Marxist-Leninist, the Nepali Congress and the Maoists - are fielding candidates for the 601-seat constituent assembly, which will also serve as a parliament.

Prakash Man Singh, a former minister and a Kathmandu-based candidate from the Nepali Congress, told AFP he expected people "to vote in huge numbers".

"Hurling petrol bombs at them will not deter them from exercising their rights," he said.

After violent anti-poll protests left one dead and injured more than a dozen others, authorities are taking no chances.

The government has deployed 50,000 soldiers and 140,000 police personnel to guard polling stations.

Security measures include a ban on all public and most private transportation, meaning that nearly all voters will have to walk to polling stations to cast their ballot.

Only ambulances and vehicles belonging to the media, diplomatic organisations, non-profits, polling officials and election observers will be allowed on the streets on Tuesday.

"All the security personnel have reached the polling stations... we have done all we could to ensure the security of the voters", police spokesman Ganesh K.C. said.

AFP