Emergency workers in India battled Thursday to rescue dozens of people still trapped after a flyover collapsed onto a busy street, killing at least 15 people and injuring over 100 more.

The flyover was under construction when a 100-metre (330-feet) section collapsed suddenly onto a crowded street in the eastern city of Kolkata around lunchtime, crushing pedestrians, cars and other vehicles under huge concrete slabs and metal.

Authorities said they expected the death toll to rise further, with an unknown number of people still trapped under the rubble.

"Fifteen people have been declared dead. More than 100 people have been rescued and are in hospital," said Anil Shekhawat, a spokesman for the National Disaster Response Force.

"Our five teams with equipment are on the spot, carrying out rescue operations."

Specialist rescue teams armed with concrete and metal cutters, drilling machines, sensors to detect life and sniffer dogs were dispatched to the scene.

But many of those engaged in the rescue appeared to be ordinary people who were seen trying to pull away concrete slabs with their bare hands.

Workers 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, where locals desperately waited for news of missing loved ones.

"Everything is finished," screamed Parbati Mondal, whose fruit-seller husband had not been seen since the accident.

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."

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

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.


'Like a bomb blast'

Many locals said they were fleeing their houses for fear that more of the damaged structure could collapse.

"We heard a massive bang sound and our house shook violently. We thought it was an earthquake," 45-year-old resident Sunita Agarwal told AFP.

"We're leaving -- who knows what will happen next."

The disaster came just days before the World T20 cricket final, which is set to draw thousands of fans to the city this Sunday.

Television footage showed one bloodied body trapped under a concrete slab, and also the hand of a person sticking out from under twisted debris.

An eyewitness at the scene described a loud bang "like a bomb blast and suddenly there was a lot of smoke and dust".

A crane was seen lifting a mangled car from under the debris and part of a crushed bus was visible protruding from the rubble, although it was unclear if it had been carrying passengers.

K P Rao, a representative of the Indian construction company IVRCL which was contracted to build the giant flyover, called the disaster an "act of God".

The firm was given an 18-month deadline and a budget of nearly $25 million to complete the project in 2009, but after seven years only about 55 percent of the work has been done.

In 2014 the company wrote to the city's development authority to say it was running out of funds to complete the project.

Mamata Banerjee, the chief minister of West Bengal of which Kolkata is the capital, told reporters those behind the disaster would "not be spared".