Maoist rebels set off a landmine under a truck on Tuesday in the Indian state of Chhattisgarh, killing 55 people and wounding at least 20 who belonged to a government-backed anti-Maoist group, police said.
The attack came on the eve of a visit to the country by US President George W. Bush and was one of the worst single acts of violence by Maoists in the past three decades.
Analysts said the attack showed New Delhi could ill-afford to take the growing Maoist threat lightly, saying that the rebels posed a bigger danger than Kashmiri Islamist militants.
Last year the local Hindu nationalist Bharatiya Janata Party government set up and started funding local anti-Maoist groups in impoverished and underdeveloped areas and provided arms to some members to fight the guerrillas.
Officials said hundreds of police reinforcements had been sent to the area to search for those behind the attacks.
Darmagura, 500 kilometres south of the state capital Raipur, is a stronghold of Maoists who claim to be fighting for the rights of peasants and landless labourers.
Indian Maoists, who operate in at least nine of the country's 29 states, have stepped up attacks in the past year, killing dozens of people, including police. India's home ministry said there are about 9,300 Maoist guerrillas operating in the country.
India's Home Ministry said last week Maoist violence has been rising, with 892 people killed in 2005, compared to 653 in 2004.
The Maoists, who often target those they consider government informers as well as landlords and local officials deemed to be corrupt, temporarily seized parts of a town in eastern Bihar state in November.