The policemen were returning to their base late Friday after inspecting security arrangements for local elections this weekend. The guerrillas have called for a boycott of the polls and have put up posters in the district ordering people not to vote.
"The land mine attack was aimed to create terror among the voters," police official Neelmani said.
The mine went off as they passed under a bridge in the Sheohar district of Bihar state, said Neelmani, who uses only one name. All six policemen in the jeep were killed.
The rebels, inspired by Chinese revolutionary leader Mao Zedong, have been fighting the government in eastern India for more than four decades, demanding land and jobs for the poor. Their influence has spread as they have tapped into anger among the rural poor who feel left out of India's economic gains.
The insurgents often target police, soldiers and government officials whom they see as representatives of the state. The rebels are now present in 20 of India's 28 states and have an estimated 20,000 fighters, according to the Home Ministry. Prime Minister Manmohan Singh has described the insurgency as the biggest internal security challenge facing the country.