Indian security forces have killed at least 13 civilians in a botched ambush near the Myanmar border, officials say.
An army patrol mistakenly opened fire on miners returning home after work, killing six. Seven more civilians and an Indian soldier died when angry locals confronted troops.
Home Minister Amit Shah said he was "anguished" and vowed to investigate.
The army has been battling separatist militants in the north-eastern state of Nagaland for years.
But it is not the first time Indian security forces have been accused of wrongly targeting innocent locals in their operations.
The incident on Saturday night took place in and around Oting village in Mon district, which borders Myanmar, during a counterinsurgency operation, reports said.
Soldiers from the Assam Rifles, an Indian army unit, opened fire on a truck carrying 30 or more coalmine labourers near their camp.
"The troopers had intelligence inputs about some militant movement in the area and on seeing the truck they mistook the miners to be rebels and opened fire killing six labourers," a senior police official told Reuters.
As news spread of the killings, hundreds of locals surrounded the camp before burning vehicles belonging to the Assam Rifles and clashing with troopers.
Soldiers then shot dead a further seven people.
The Indian army said in a statement that one of its soldiers was killed in the clash, while others were injured.
"The cause of the unfortunate loss of lives is being investigated at the highest level," it said.
Following the incident, on Sunday evening protesters threw stones at the Assam Rifles camp and tried to set fire to parts of it, local media reported.
Nagaland's chief minister called the killings "highly condemnable" and said they would be investigated.