KATHMANDU (Reuters) - Nepali police shot dead at least four protesters and killed a fifth on Tuesday as demonstrators threw stones and petrol bombs, angry at a new planned constitution.

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.

Protesters in Nepal's southern plains have been agitating for weeks against plans to divide the small area into several provinces, part of an overhaul envisaged under a federal constitution that politicians are now finalizing.

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.

Police official Kamal Singh Bam said police shot and killed four demonstrators in the town of Birjung inParsa district, 60 km (40 miles) south of Kathmandu, when a police post was attacked with stones and petrol bombs.

In a separate clash with police, a fifth demonstrator was killed in the neighboring district of Bara, police official Lokendra Malla said, without giving 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 - demand that their narrow strip of homeland should not be divided into more than two states.

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 of a new president, prime minister and speaker.