Maoists have a presence in more than 200 districts of India
At least 17 policemen including a top commander have been killed in a battle with Maoist insurgents in the Indian state of Maharashtra, officials say.
They said the fighting started after a group of Maoists attacked a police station in Gadchiroli district.
It was not immediately clear whether the rebels suffered any casualties.
India says the Maoists pose its biggest security threat. They operate in many states and say they are fighting for the rights of the poor and landless.
Prime Minister Manmohan Singh told police chiefs last month that a campaign against the rebels had failed to produce results.
Thousands of people have died in the insurgency they launched in the 1960s.
Last weekend the rebels killed 16 villagers in the state of Bihar. On Tuesday the body of a policeman kidnapped by the rebels was found beheaded in the state of Jharkhand.
The rebels killed 16 police personnel in a gun battle in Gadchiroli district in May. Fifteen officers were killed in the same district in February.
Police said that in the latest attack, at least 150 Maoists attacked policemen in a forested area for more than two hours.
"One senior officer and 16 constables have died," police inspector SD Mundhe told the AFP news agency.
"The rebels have fled to the Chhattisgarh border," Mr Mundhe said, in a reference to the neighbouring central Indian state where the rebels have their stronghold.
"We cannot search for them in the dark."
On Wednesday Home Minister P Chidambaram warned the rebels to abandon violence or face a major assault by security forces following the beheading of the police officer.
In September Kobad Ghandy, one of the Maoists' most senior leaders, was arrested in Delhi. A court in Delhi has remanded him in custody for another week.