The attack has been blamed on Maoist rebels, who operate in the area as well as several other Indian states.
It is one of the worst attacks on Indian police in two years.
Maoist rebels in the eastern state of Orissa kidnapped two Italians about two weeks ago, but released one and have been in talks with the state government over the other man’s release.
Tuesday’s attack happened in Maharashtra’s Gadchiroli district, a remote and relatively undeveloped part of the country.
The police vehicle was travelling through a forested region when the landmine was triggered, reports said.
The policemen, members of India’s national paramilitary force, were travelling on a bus, the Press Trust of India news agency quoted officials as saying.
The explosion was so powerful that it could be heard several kilometres away and the bus was turned into a mangled heap of twisted metal, a police officer in the district told the BBC.
The Maoists control large areas of several states in a “red corridor” stretching from the north-east to central India.
They say they are fighting for communist rule and greater rights for tribal people and the rural poor.