At least four protesters have been killed and four others injured in Nepal after security forces opened fire at a political rally, according to police.

Thousands of officers were deployed on Monday to the southeastern town of Rajbiraj, where the Unified Marxist and Leninist party had organised a get-out-the-vote rally before local elections set for May 14.

Members of the Madhesi ethnic community, who oppose the upcoming polls, held a counter-protest. The police opened fire at the protesters when they began hurling stones at the leftists.

An 18-year-old man died at a local hospital, while three others died at another hospital in Dharan, a town about 80km east, said Sarbendra Khanal, a spokesman for Nepal Police.

"The area is still tense. Protesters have set at least four vehicles on fire and have obstructed the highway," Khanal said, adding that about 40 security forces were injured in the clashes.

Khanal said that the leaders of the party that organised Monday's rally were escorted back to their homes in the east of the country.

The opposition party has been critical of the protesters' demand seeking greater rights.

Nepal's coalition government announced last month that polls would be held on May 14 for more than 700 local bodies, the first local elections in 20 years.

The government's move has met fierce opposition from the Madhesis, who protested by shutting down the southern region bordering India for a day.

The polls were delayed for years because of civil war and political infighting, but the protesters have refused to participate in the elections and are demanding amendments to the country's new constitution.

The Madhesis say the constitution discriminates against them by limiting their representation in state institutions.

They have long called for the redrawing of provincial boundaries to ensure greater representation for their community.