Markets, schools and factories were closed across Nepal's southern plains on Tuesday in response to the killing of three protesters who tried to disrupt a political rally.

Security forces opened fire at the rally on Monday, killing three protesters and wounding more than a dozen, in the south-eastern town of Rajbiraj, where the Unified Marxist-Leninist (UML) party had organised a get-out-the-vote rally before local elections.

The police put the death toll at four, but later revised it to three.

Santosh Niraula, a local police officer, said much of the public transport remained off the roads in the southern flatlands bordering India.

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The Samyukta Loktantrik Madhesi Morcha, an alliance of seven regional parties that organised Monday's protest, called for a two-day general strike in the region.

The group has accused leaders of the UML, the country's main opposition party, of humiliating its supporters and deriding the community during the mass rally.

The protesters 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.

Thousands of police officers were deployed on Monday to maintain security at the UML rally, but clashes still erupted outside the venue where hundreds of protesters waved black flags and blocked roads.

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