Plans to complete long-delayed constitution come as many Nepalis focus on rebuilding following devastating earthquakes.
Police have killed at least five protesters as demonstrators threw stones and petrol bombs, angry at a new planned constitution.
Kamal Singh Bam, a police official, said on Tuesday that officers had shot and killed four demonstrators in the town of Birjung in Parsa district, 60km south of the capital Kathmandu, when a police post was attacked.
In a separate clash with police, a fifth demonstrator was killed in the neighbouring district of Bara, police official Lokendra Malla said, without giving further details.
Under the constitutional proposals, 22 districts in the southern plains, also known as the Tarai, would be joined with provinces that are dominated by mountain dwellers.
The protesters, mostly from the Madhesi and Tharu minority communities, are demanding that their narrow strip of homeland should not be divided into more than two states.
More than 20 people have died in protests since those plans were unveiled two weeks ago, with members of two large plains communities demanding greater autonomy under the charter.
The government and major political parties hope that the constitution, in the works for seven years, will provide much-needed political stability and bolster economic development in the Himalayan nation, which is still reeling from two devastating earthquakes that killed 8,900 people this year.
Prime Minister Sushil Koirala has called for talks to tackle the problem, but the protesters insist that the constitutional process must be stopped before any dialogue begins.
Adoption of the charter, which requires a two-thirds majority in parliament, would be followed by elections for a new president, prime minister and speaker.