At least 25 people have died in a gun battle between former Maoist rebels and supporters of a regional rights group in southern Nepal, police say.
The Madheshi community wants a federal system
More than 30 others were wounded in the fighting in and around the town of Gaur. A curfew has now been declared.
Maoists and protesters seeking more rights for the Madheshi people in the south have spent months at loggerheads.
The bloodshed is the worst Nepal has seen since a truce last year which led to peace accords with the Maoists.
Local police chief Ram Kumar Khanal told the BBC that clashes broke out over a meeting ground in Gaur which both sides wanted to use.
Many Maoists have kept their weapons despite the peace deal
Thousands of opposing supporters of the two groups converged on the same venue.
The United Nations says two women were among those killed. Many of those hurt are said to have serious head injuries.
Reports say batons were used in the clashes and many rounds of gunfire were heard.
A spokesman for the Madheshi Janadhikar Forum (MJF), one of the groups campaigning for greater rights for people in the south, told the BBC that up to 50 people had been killed in clashes in the area.
It was not possible to independently confirm the figure.
The BBC's Charles Haviland in Kathmandu says it is the worst day of violence since a ceasefire nearly a year ago which led to peace accords with the Maoists.
Groups speaking for Madhesis, or southern Nepalese, have been demonstrating since December for greater rights under the country's new constitution.
The Maoists are bitterly opposed to most of these organisations, saying they themselves are the best guarantors of regional and ethnic rights.
Under peace accords, thousands of former Maoist fighters and their weapons are supposed to be confined in UN-approved cantonments.
Separately, Nepal's business community has continued a programme of protests against what they say is an incessant Maoist campaign of violence, extortion and intimidation.
Maoist leaders have however been meeting leading business figures to try to assure them this will stop.
The indefinite strike called by business owners has disrupted normal life across the country, although shops opened and vehicles have returned to the streets in Kathmandu.
After a peace agreement last November that effectively ended Nepal's 10-year-old insurgency, Nepalese parties are currently preparing for polls in June to elect an assembly that will write a new constitution.
But experts are worried that the series of strikes could delay the Maoists joining an interim government that will hold the elections.
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