oters in Nepal's northern Himalayan districts were set to cast their ballots in the first phase of national and provincial assembly elections on Sunday, two years after Nepal adopted a new constitution establishing a federal democracy.

The polls are a major step toward implementing the new constitution, which aims to empower marginalized communities, including women and Dalits - the so-called untouchables - by ensuring greater representation in state institutions.

A close race between the ruling Nepali Congress and an alliance of the Unified Marxist-Leninists (UML) with the Maoists is expected in the polls.

A total of 275 seats in the new federal parliament and 550 seats in seven provincial assemblies are up for grabs across the country.

Nepal sealed its northern border with China during the elections for security reasons. It has also banned traffic from roads to prevent vote-rigging.

But a series of explosions targeting candidates has marred the election campaign.

Police have arrested more than 500 cadres from the Communist Party of Nepal, a splinter Maoist group accused of carrying out explosions, though the outfit hasn't claimed responsibility for the attacks.

On Saturday morning, a crude bomb exploded at the entrance to the home of former Prime Minister and UML chairman K.P. Sharma Oli in the eastern town of Damak. Oli was away for a campaign.

Votes will be counted following the second phase of polls held on December 7 in the capital Kathmandu and the low-lying areas in the south of the country.

The polls are expected to cap a peace process that began with the end of a decade-long civil war in 2006. The war pitted the Maoists against the state and left more than 16,000 people dead.