KATHMANDU (Reuters) - Nepali police shot and killed at least three ethnic Madhesis in the country’s restive southern plains as they tried to disrupt an opposition rally, officials said on Monday, the deadliest incident in more than a year over its post-monarchy charter.

The volatile Himalayan nation, sandwiched between China and India, has been in turmoil since the September 2015 adoption of the constitution that the minority Madhesi people living along the border with India oppose for failing to accommodate their interests.

More than 50 people died in clashes in 2015 and there were severe shortages of fuel and medicines due to violence at the Indian border. The protests ended after the government promised to amend the constitution to address the grievances of the Madhesis, though no changes have been made yet.

Fresh trouble started on Monday in Rajbiraj, 150km (90 miles) southeast of capital Kathmandu, after Madhesi activists tried to storm a public rally organized by the main opposition Communist Unified Marxist-Leninist (UML) party that opposes any change to the charter.

“We have three people confirmed killed,” Home Ministry spokesman Bal Krishna Panthi told Reuters.

Local television channels said four people had been killed.

Police spokesman Sarbendra Khanal said the Madhesi protesters tried to storm the UML rally despite the use of water cannon, baton charge and teargas shells to disperse them. More than two dozen people, including police personnel, were injured, he said.

Hridayesh Tripathi, a senior leader of the Tarai Madhesh Loktantrik Party, blamed the UML for ignoring its request to cancel the rally and increasing tension in the region.

With the landlocked nation of 28.6 million people recovering from its worst earthquake on record nearly two years ago, Nepal has struggled to complete its political transition after years of civil war.

The incident in Rajbiraj could dim prospects for elections to local bodies which the Madhesis have vowed to boycott without their grievances being addressed. Prime Minister Prachanda has said he was committed to changing the constitution before the voting, which has been set for May 14.

Madhesis say the entire southern plain region, Nepal’s bread basket, must not be split into more than two federal provinces. It now forms part of six of the seven states dominated by hill dwellers.