The death toll from a collapsed flyover in eastern India rose to 25 on Friday, after emergency workers toiled through the night to find any remaining survivors trapped under huge slabs of concrete and metal girders.

But emergency authorities said there was little hope of finding any more survivors after pulling almost 100 people from under the rubble of the road that collapsed onto a busy street in Kolkata on Thursday, crushing cars and pedestrians.

"The rescue operation will not stop until all the blocks of concrete and iron girders have been cleared," said deputy police commissioner Akhilesh Chaturvedi as he announced the toll had risen to 25.

"Nearly 300 rescuers, including army and disaster management personnel, are working around the clock to clear the rubble."

The National Disaster Response Force (NDRF) however said there was little hope of finding any more survivors under the rubble of the flyover, which had been under construction since 2009.

It was unclear what caused the sudden collapse, but police have registered a preliminary case against the contractor.

The state government, which is fighting for re-election in a vote that starts on Monday, pledged to take action against those found responsible.

Survivors being treated at a nearby hospital described how tonnes of metal and concrete came crashing down onto the busy street without warning.

"The flyover collapsed in front of me. When I tried to escape, I was hit," said housewife Sabita Devi.

Hospital manager Sitaram Agarwal said many people were being treated for head and leg injuries sustained in the disaster.

But authorities initially struggled to get cranes and other large machinery through the narrow streets of Burrabazar, one of the oldest and most congested parts of the city.

'GROSS NEGLECT'

An injured builder told AFP at the scene that he had been working on the structure before it collapsed and had seen bolts come out of the metal girders.

"We were cementing two iron girders for the pillars, but the girders couldn't take the weight of the cement," said 30-year-old Milan Sheikh before being taken away to hospital.

"The bolts started coming out this morning and then the flyover came crashing down."

The disaster is the latest in a string of deadly construction accidents in India, where enforcement of safety rules is weak and substandard materials are often used.

Construction of the two-kilometre-long flyover began in 2009 and was supposed to be completed within 18 months, but has suffered a series of hold-ups.

Mamata Banerjee, the chief minister of West Bengal state, of which Kolkata is the capital, said the government "will not spare" the contractors, Indian construction company IVRCL.

Police said they had registered preliminary cases of murder, attempt to murder and criminal conspiracy against the company, whose offices in Kolkata have been sealed by investigators.

Earlier KP Rao, a representative of IVRCL, appeared to deny any responsibility for the disaster when he told reporters it was an "act of God".

The Times of India said it was "another brutal reminder of (the) shoddy quality of construction and gross neglect of public safety in our cities", calling for a thorough enquiry to determine what went wrong.

The disaster comes at a sensitive time for Banerjee, whose centre-left Trinamool Congress party is seeking re-election in the state.

Voting in the West Bengal elections begins on Monday and will be held in five phases lasting a month.

On Thursday, Banerjee blamed the previous state government under which the flyover project was started.