Maoist rebels have killed at least 75 Indian soldiers in a series of attacks on security convoys in the central state of Chhattisgarh, officials say.
A large patrol of federal paramilitary troops was ambushed at dawn by hundreds of heavily armed insurgents in a remote part of Dantewada district.
Rescue teams were later ambushed in attacks using landmines and gunfire.
Correspondents say it is the worst attack on security forces by the rebels since their insurgency began.
India's Home Affairs Minister P Chidambaram said the attack showed the brutality and savagery that the rebel army was capable of.
But he suggested lessons had to be learnt quickly by the security forces.
"Something has gone very wrong. They seem to have walked into a trap set by the Naxalites [Maoists]. Casualties are quite high and I am deeply shocked," he said.
Home Secretary Gopal K Pillai said that the rebels had booby-trapped the area of the ambush.
"Preliminary reports indicate that the Maoists planted pressure bombs in surrounding areas at places where the security forces might take cover," he said.
"As a result of this, the bulk of the casualties have arisen from the pressure bomb blasts."
The Maoists have stepped up attacks in recent weeks in response to a big government offensive along what is known as the "red corridor", a broad swathe of territory in rural eastern and central India where the Maoist rebellion has been gathering strength.
Nearly 50,000 federal paramilitary troops and tens of thousands of policemen are taking part in the operation in several states.
The rebels have tapped into rural and tribal anger among those who have seen no benefits from India's economic development and this attack is another chilling reminder of the growing threat they pose, says the BBC's Chris Morris in Delhi.
Thousands of people have died during the rebels' long fight for communist rule.
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.