Indian forces in the north-eastern state of Nagaland killed at least 13 civilians in an ambush near the border with Myanmar, 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 Nagaland for years.
The incident on Saturday night took place in and around Oting village in Mon district, which borders Myanmar, during a counterinsurgency operation, Reuters reported.
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,” the official said, requesting anonymity as he was not authorised to speak with the media.
As news spread of the killings, hundreds of locals surrounded the camp before burning vehicles belonging to the Assam Rifles and clashing with troopers using “crude weapons”, he said.
Soldiers then shot dead a further seven people, state police officer Sandeep M Tamgadge told AFP.
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.
Mr Shah expressed his “deepest condolences” to the families of those killed and promised to “ensure justice”.
Nagaland’s chief minister called the killings “highly condemnable”.