BHUBANESWAR, India, April 13 (Reuters) - Maoist rebels killed 10 policemen in eastern India and shut down a mine operated by state-run National Aluminium Co Ltd (NALCO) <NALU.BO> as they stepped up attacks ahead of a general election.

Police said at least 100 rebels attacked a NALCO bauxite mine guarded by police in the mineral-rich state of Orissa late on Sunday.

Four Maoists were also killed during a heavy exchange of fire between the rebels and police, Deepak Kumar, a senior police officer said.

The company said the open pit mine, with a deposit of more than 310 million tonnes of bauxite reserve, would be shut for at least a day as a result of the attack.

The area also houses the company's depot, where a huge quantity of explosives meant for quarrying are stored.

"They attacked during the night shift and trapped nearly 60 employees," senior NALCO official P.K. Mohapatra, told Reuters. The rebels fled after more police reached the area.

The rebels have stepped up attacks in eastern and central India ahead of a general election in April and May, which they are boycotting.

The Maoists, who say they are fighting for the rights of poor farmers and landless labourers, have threatened to chop off hands of voters if they went to polling booths.

Thousands have been killed in the Maoist insurgency which began in the late 1960s and now stretches throughout rural areas of eastern, central and southern India.

Prime Minister Manmohan Singh has described the insurgency as one of the gravest threats to India's internal security. (Reporting by Jatindra Dash; Editing by Bappa Majumdar)