"About 500 armed rebels rushed to Nayagarh town in vehicles and attacked a police station, armoury and police training centre, hurled bombs in several places and looted huge quantity of arms and ammunitions," Kumar said.
Another 12 policemen were wounded in the assaults.
Gopal Chandra Nanda, Orissa's director general of police, said hundreds of police and paramilitary forces are searching for the fighters, known as Naxalites.
The Press Trust of India news agency reported that the fighters took away the stolen weapons in a bus that they hijacked earlier.
However, Nanda was unable to confirm the report.
The Naxalites, who say they are fighting for the rights of the poor and landless, regularly stage raids in eastern and central India where they have a presence.
They are called Naxalites after Naxalbari, a village in West Bengal state where the movement was born in 1967.
Manmohan Singh, the Indian prime minister, has said the Naxalites are the gravest threat to India's internal security.