The blast blew the bus to pieces
Maoist rebels have attacked a bus in central India and killed at least 20 people, including civilians and police officers, according to officials.
The rebels apparently detonated a mine under the bus in Chhattisgarh state's Dantewada district. Some reports put the death toll at much higher than 20.
Dantewada was the scene of the rebels' deadliest attack last month, when 76 people were killed.
Thousands of people have died in their decades-long fight against the state.
In the latest attack, the rebels are said to have destroyed the front of the bus with a landmine.
INDIA'S MAOIST INSURGENCY
Violent rebellion began in 1967 in West Bengal village of Naxalbari and spread over rural areas of central and eastern India
Led by elusive military commander Kishenji, supported by between 10,000 and 20,000 fighters
More than 6,000 killed since rebellion began
Worst attacks include 76 killed in April 2010 ambushes in Dantewada; 55 killed in attack on police outpost in 2007
"About 40 people were travelling on the bus and at least 20 police personnel and a couple of civilians have been killed in a landmine blast," local official SR Kalluri told the AFP news agency.
He said the officers were special police officers, who are recruited from the civilian population to act as pro-government militias, helping security forces in anti-Maoist operations
Local TV stations reported that as many as 50 people may have died in the attack.
The BBC's Chris Morris in Delhi says questions will be asked about why members of the security forces were travelling on a civilian bus in such a dangerous area.
He says officials will also want to know how the rebels managed to find out about it so quickly and mount an attack.
Earlier on Monday, the bodies of six villagers were found with their throats slit in the forests of Chhattisgarh.
Maoist rebels had kidnapped the six on the weekend, accusing them of spying for the government.
India's 'biggest threat'
Chhattisgarh has seen several attacks recently, and Dantewada is one of the areas hardest hit by the insurgency.
Last month, the district was the scene of the worst attack the rebels have yet carried out with the killing of 76 people, including soldiers and police officers.
The Maoists, also known as Naxalites, say they are fighting for the rights of rural poor who have been neglected by the government for decades.
But correspondents say support has been slipping away from the rebels, particularly after last month's attack.
The Maoist insurgency has been described by Prime Minister Manmohan Singh as the country's biggest internal security threat.
Last October, the government announced a "massive anti-Maoist offensive" and deployed 50,000 troops in several states, including Chhattisgarh.