At least 145 people were injured - many critically - when two trains collided overnight in West Bengal.

Police said Maoist rebels sabotaged the track causing the Calcutta-Mumbai passenger train to derail, throwing five of its carriages into the path of an oncoming goods train.

But a spokesman for the Maoist rebels called the BBC to deny any involvement.

A railway spokesman said 78 bodies had been recovered, but that more than 30 remained trapped in three carriages that were crushed by the freight train.

Officials said rescue workers were still working to free survivors from the wreckage in the Jhargram area of West Midnapore, about 150km (90 miles) west of Calcutta.

The area is known to be a stronghold of Maoist rebels.

The state police chief, Bhupinder Singh, said a 46cm (1.5ft) portion of the train track was missing.

He said posters from a local group believed to have close ties to the Maoists were found at the scene, claiming it had carried out the attack.

But Maoist spokesman Comrade Khokan spoke to the BBC by telephone to deny any involvement. He said the government "put the blame on us and put us on the defensive".

Some analysts say that in this case the Maoist script may have gone awry.

Maoists frequently tamper with railway lines and often these lead to minor derailments; there have been hijackings but no major attacks on civilian transport with such a death toll.

The rebels had called for observance of a "black week" of protests from Friday to Wednesday in five states - including West Bengal - where they wield considerable influence

The incident happened at 0130 local time (2000 GMT) when it was still dark, which meant the driver had failed to see the damaged track, rail officials said.

Rescue workers were still using sledgehammers and gas cutters to break into carriages where survivors were thought to be trapped, the BBC's Amitabha Bhattasali reported from the scene.

Bodies are still being removed from mangled carriages and helicopters are airlifting injured passengers to nearby hospitals.

Hundreds of police, troops and emergency workers are involved in the search and recovery effort.

Railway spokesman Soumitra Majumdar said five coaches of the passenger train, the Gyaneshwar Express, had been derailed due to missing "fish plates" - which join rails together.

These coaches then fell on to the neighbouring track where they were rammed by the goods train, he said.

There were 13 carriages - including 10 sleeper coaches and a coach with unreserved seating - on the passenger train, the Times of India reported.

Railway Minister Mamata Banerjee has visited the crash site.

The government has been under pressure following a wave of Maoist-led violence.

In April, 76 paramilitary troops were killed in an ambush - the single biggest attack on the Indian security forces by the rebels.

Maoist rebels have in recent months stepped up attacks in response to a government security push to flush them out of their jungle bases.

Prime Minister Manmohan Singh has described the insurgency as India's biggest internal security challenge.