At least six Indian police officers were killed on Sunday when Maoist rebels detonated a bomb under their vehicle.
Another officer was critically injured in the attack in Dantewada district of Chhattisgarh state.
D M Awasthi, the chief of counterinsurgency operations in the state, said the police were escorting a truck carrying material to a road construction site in a forest area when the rebels triggered the blast.
Five policemen died on the spot and one of two officers who were critically injured died later in hospital, police said.
Police and paramilitary soldiers have launched a hunt for the attackers.
India's Maoist rebels, inspired by Chinese revolutionary leader Mao Zedong, have been fighting the government for more than four decades, demanding land and jobs for tenant farmers, the poor and indigenous communities.
The government has called the rebels India's biggest internal security threat. With thousands of fighters, the rebels control vast swathes of area in the country.
Also known as Naxalites, the rebels have ambushed police, destroyed government offices and abducted government officials. The insurgency began in 1967 as a network of left-wing ideologues and young recruits in the village of Naxalbari outside Kolkata, the capital of West Bengal state.
They have blown up train tracks, attacked prisons to free their comrades and stolen weapons from police and paramilitary warehouses to arm themselves.
Last month, authorities said troops killed at least 44 suspected rebels in multiple raids in western India.