The clash erupted after about 20,000 demonstrators defied 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 breached the restricted zone in Tikapur, killing eight policemen, including an SSP, the spokesperson said.
"The number of casualty may rise and there might be some civilian casualty also, but detail report has not yet come," the official said.
The demonstrators have been protesting against the draft charter, saying the proposed provinces fail to ensure political representation for marginalised communities, including the Madhesis.
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The protesters claim that the new borders will discriminate against historically marginalised communities.
Home Minister Bam Dev Gautam said the government would send troops to the area to try to bring the situation under control, and appealed for peace.
Additional police force has been dispatched to the incident site for controlling the situation.
Since 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.