Maoist rebels have killed at least 72 Indian soldiers in a series of attacks on security convoys in the central state of Chhattisgarh, officials say.
Troops were on patrol in dense jungle in a remote part of Dantewada district when rebels opened fire on them and set off explosives, police say.
Rescue teams were also ambushed. Police say fighting is continuing.
It is the biggest loss of life security forces have suffered since launching a recent offensive against the rebels.
Nearly 50,000 federal paramilitary troops and tens of thousands of policemen are taking part in the operation in several states.
Thousands of people have died during the rebels' 20-year fight for communist rule in large swathes of rural India, known as the "red corridor".
'Trap'
Details of the attacks in Dantewada district remain sketchy.
Police say the rebels initially attacked a convoy of the paramilitary Central Reserve Police Force (CRPF) in the Talmetla area.
However, India's Home Minister P Chidambaram said it appeared that the forces had "walked" into a rebel ambush.
"Something has gone very wrong. They seem to have walked into a trap set by the [Maoists] and casualties are quite high," he said.
RK Vij, a spokesman for the CRPF, told the BBC that 67 bodies of security personnel had been recovered from the site of the fighting.
"The injured troops have been evacuated by helicopter. More reinforcements have been sent," Mr Vij said.
The rebels also attacked troops sent to rescue their colleagues, police said.
"Fighting is still carrying on in the area, and we're having great difficulty getting news from there," police official Ashok Dwivedi told the Reuters news agency from Chhattisgarh state capital, Raipur.
The BBC's Sanjoy Majumder in Delhi says Chhattisgarh is a major stronghold of the Maoists whbo control large swathes of territory in central and eastern India.
Talks call
The latest attacks come two days after rebels killed at least 10 policemen and injured 10 more in a landmine attack on a police bus in the eastern state of Orissa.
The rebels say they will step up attacks unless the government halts its offensive against them.
Mr Chidambaram has said troops will intensify the offensive if the rebels do not renounce violence and enter peace talks.
The Maoists want four senior leaders freed from jail and the offensive halted before any talks.
Prime Minister Manmohan Singh has described the Maoist insurgency as India's "greatest internal security challenge".
The Maoists say they are fighting for the rights of the rural poor who they say have been neglected by governments for decades.
ANALYSIS
By Soutik Biswas
BBC News, Delhi
The attack on troops in Chhattisgarh is the deadliest since they began their latest anti-Maoist operation.
Although details are still sketchy, it is clear it will not be easy for the security forces to defeat the rebels in their strongholds - vast swathes of remote mineral-rich jungles home to tribespeople who form the main support base for the rebels.
Questions have been asked about levels of intelligence and whether security forces are trained to operate in such hostile terrain.
The attack is also a blow to the government - it comes days after Home Minister P Chidambaram described the rebels as "cowards enacting dramas". With the prospect of dialogue unlikely, it appears the government is in for a long and difficult war.
BBC News
