Four people were killed and nearly 30 wounded when a bomb attached to a motorbike went off near a crowd of protesters in southern Nepal on Monday, police said.
The group of 150 demonstrators in the city of Janakpur, 20 kilometres (12 miles) from the Indian border, were staging a sit-in to demand a separate province for Maithili-speaking people when the bike exploded.
"One of the protesters died on the spot while three died on their way to hospital. We have referred around seven seriously injured victims for treatment to Kathmandu," local police chief Basanta Raj Gautam told AFP.
"Over 20 people who sustained minor injures from the blast are being treated at various hospitals in the district."
The protesters had been taking part in a strike in Janakpur to demand the new province in the southern Terai plains. The Maithili language is spoken in eastern India and southeast Nepal.
Nepal has been relatively peaceful since rebel Maoists waging a 10-year war against the government signed a peace accord in 2006.
The country's parliament is nearing a deadline to write a peacetime constitution which will divide the country into new federal states.
"There were around 150 persons sitting in the protest... when the bomb went off. We have increased security in the area but no one has taken responsibility for the incident," Gautam said.
"The strike was called by the Mithila State Struggle Committee for three hours today and their cadres had blocked road transportation to pressure for a separate Mithila province in the constitution."
Uma Shankar Singh, a sub-inspector of police, said the bomb had been fitted on the tail box of the motorcycle. No one has been arrested in connection with the attack.
A powerful blast in Kathmandu in February killed three people and wounded seven in the first such incident in the capital for three years.
An organisation calling itself the United Ethnic Liberation Front (UELF) claimed responsibility, although their motives were unclear.