Indian police said on Friday they have detained five staff at the company building a flyover that collapsed, killing at least 25 people, as hopes of finding more survivors faded.

Emergency workers using specialist equipment have pulled nearly 100 survivors from under the huge concrete slabs and metal girders that fell onto a busy street in the eastern city of Kolkata on Thursday, crushing cars and pedestrians.

On Friday they said there was little hope of finding any more survivors under the rubble of the flyover, which had been under construction since 2009.

"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."

Police said they had detained five employees of IVRCL, the contractor behind the construction project, which has denied responsibility for the disaster in the capital of West Bengal state.

"Five people of the Hyderabad company have been detained for questioning," said Kolkata deputy commissioner of police Akhilesh Chaturvedi, referring to the contractor.

He gave no further details, but police earlier said they had registered a case of culpable homicide against the firm, whose offices in Kolkata have been sealed by investigators.

Derek O'Brien, a state lawmaker, said the company had been blacklisted in other states and had a "bad reputation".

"The law will take its own course, no one will be spared," he told reporters in Kolkata.

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.

A company representative infuriated victims on Thursday when he described the disaster as an "act of God".

On Friday the company appeared again to deny any responsibility for the disaster, and said the construction had been repeatedly delayed because it had been unable to get the necessary approvals.

"Prima facie we feel it is a mere accident for which we also feel very, very sorry," IVRCL's legal chief Seetha Peddapathi told reporters in the southern city of Hyderabad where the company is based.

"IVRCL and its staff will cooperate with the investigations and provide maximum support."

'Gross negligence'  

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.

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.

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.

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 inquiry to determine what went wrong.

The disaster comes at a sensitive time for West Bengal's Chief Minister Mamata Banerjee, whose centre-left Trinamool Congress party is seeking re-election.

Banerjee has blamed the previous state government under which the flyover project was started, but has herself faced criticism over the beleaguered construction project.

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