Thousands of people had come out onto the streets in defiance Monday, encroaching a "prohibited zone" imposed by the administration since Friday.
A mob surrounded officers in the district of Kailali, in the far west of Nepal as officials attempted to enforce restrictions. The protesters used axes, scythes and spears to attack the officers, Raj Kumar Shrestha, chief district officer of Kailali District, told CNN.
A child was also killed during the attack, Shrestha said. There were no reports of protester casualties.
The proposed changes to the constitution -- which has been under review since 2008 following a Maoist insurgency which deposed the country's monarchy -- would federalize the country, dividing it into seven provinces.
Members of the country's Tharu minority have been vocal in their opposition to the plan, which they say would see the group further marginalized. Minorities across the country have been protesting the proposal, demanding their own provinces.
Police officer Ram Bihari Tharu was also burned alive by the protestors, Home Minister Bam Dev Gautam told parliament. He characterized the attack as pre-planned and said that the government would deploy military forces to keep order.
Sixteen other officers were injured in the attack. They're currently undergoing treatment at Nepalgunj Medical College in Banke, and authorities are preparing to airlift them to the capital.
The brother of Nepal Police Senior Superintendent Laxman Neupane, one of the slain officers, told CNN preparations were underway to perform his last rites in Kathmandu, and that the remains of all officers killed were being airlifted from Kailali to Kathmandu Tuesday afternoon.
Nepal Police Chief Upendra Kant Aryal, meanwhile, vowed to bring the killers to justice, and called upon all security personnel to remain alert and diligent in their duties.
The curfew was extended indefinitely following the attack in Kailali.