The clash erupted after about 20,000 demonstrators from the Tharu ethnic group breached the prohibitory orders and attacked police with weapons in Tikapur in Kailali district on the issue of dividing the country in seven provinces, a Home Ministry spokesperson said.
The situation turned violent after the demonstrators surrounded police who were enforcing a curfew in Tikapur and attacked security officials with bricks, spears and knives, killing six policemen, including an SSP, the spokesperson said.
The injured were taken to regional hospitals and the death toll was expected to rise, they said.
Some local media reports put the death toll at 20 that include 17 policemen. However, there was no official confirmation.
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Additional police force has been deployed at the incident site for controlling the situation.
Home Minister Bam Dev Gautam said the government has also sent troops to the area to try to bring the situation under control.
Demonstrators have been protesting against the draft charter, saying the proposed provinces fail to ensure political representation for marginalised communities.
The protesters from the Tharu ethnic group are demanding a separate state in the new Constitution.
Since the last two weeks, Tharuhat United Struggle Committee has been staging agitation in the western Terai region demanding autonomous Tharuhat province including Kanchanpur and Kailai districts in the new federal structure.
The agitating group has rejected the seven province model of federal system announced by the major political parties in June after April's devastating earthquake.