Kathmandu, Nepal (CNN) -- At least four people were killed in a bomb explosion near a political rally in southeastern Nepal on Monday, authorities said.
Officials said 18 people were injured in the blast, which occurred in the holy city of Janakpur, about 250 kilometers (155 miles) southeast of Kathmandu.
State-run Nepal Television reported that one person was arrested on suspicion of involvement and is being treated at a local hospital.
Local authorities said no one claimed responsibility for the blast. But in a text message to local media, an armed group took responsibility. Janatantrik Terai Mukti Morcha (the Democratic Terai Liberation Front) demanded that regional parties in southern Nepal stand up for the rights of people in the south.
"Initial reports say that a bomb went off in a motorcycle near a sit-in program at a market square," chief district administrator Basanta Raj Gautam of Dhanusha district said from the blast site.
Some of the more seriously injured have been airlifted to Kathmandu for treatment, he said.
The victims were at a sit-in demanding the creation of a new province as part of negotiations over the nation's constitution, according to local media reports.
The deadline for a new constitution is less than a month away.
Various groups have been demanding provinces based on ethnicity, while some political parties want provinces demarcated on the basis of economic viability.
A few years ago, several armed political groups were active in Nepal's Madhesh region, as the southern part is known, but there had been no such incidents lately.