NEW DELHI Maoist rebels killed at least 73 police after an ambush in central India on Tuesday, just the latest in a series of attacks on security personnel.
The rebels, estimated around 20,000, hold sway over vast swathes of the countryside, many rich in metals and minerals.
India's Congress party-led government is taking on the rebels in a biggest-ever offensive, reflecting growing concerns that the rebels are becoming stronger in the decades-long insurgency.
Here are some questions and answers about the Maoists and their threat in India:
WHO ARE THE MAOISTS?
They started an armed struggle with a peasant revolt in Naxalbari village in West Bengal state in 1967 but were initially crushed by the Congress-led government.
After regrouping in the 1980s, they began recruiting hundreds of poor villagers, arming them with bows and arrows and even rifles snatched from police and government armouries.
Indian authorities say they are led by Koteshwar Rao, alias Kishanji, who is in charge of militant activities, and Ganapati, the political leader. They remain hidden in dense forest bases and move around villages in remote areas.
HOW BIG IS THE MOVEMENT?
The rebels have an estimated 20,000 combatants, including up to 6,000-8,000 hardcore fighters.
Maoist rebels have made inroads in nearly a third of the country's 630 districts, according to the government.
WHERE ARE THEY ACTIVE?
They operate across a "red corridor" stretching from the southern state of Andhra Pradesh to the central state of Chhattisgarh and into West Bengal, bordering Nepal and Bhutan.
HOW DO THEY GET ARMS?
Mostly from raiding police bases. The government says the rebels also buy weapons from Chinese smugglers and are in touch with other militant groups operating in India, including groups in Kashmir and the northeast. They are equipped with automatic weapons, shoulder rocket launchers, mines and explosives.
HOW BIG A THREAT ARE THEY TO INDIA'S STABILITY?
Prime Minister Manmohan Singh has described the insurgency as the biggest internal security challenge since independence. More than 1,000 attacks were recorded in 2009 and 600 people were killed. The Maoists regularly attack railway lines and factories, aiming to cripple economic activity.
The rebels extort about $300 million from companies in India every year to fund their movement.
(Editing by Alistair Scrutton and Sugita Katyal)