An immense rescue effort is underway in the Indian city of Kolkata after a road bridge under construction collapsed into traffic, killing at least 20 people and leaving nearly 100 others injured.

Key points: Dozens hospitalised and dozens more feared trapped

Dozens hospitalised and dozens more feared trapped Construction company denies responsibility, says collapse was "an act of God"

Construction company denies responsibility, says collapse was "an act of God" Indians incensed by denial, urban planner blames "human error", lack of "system of liability"

A police complaint has already been filed against the construction company but it has denied responsibility for the incident, incensing the nation by describing the collapse as an act of God.

"So far 20 people are confirmed dead. The toll is likely to rise as many people are injured," Javed Ahmed Khan, Disaster Management Minister for the state of West Bengal, said.

It remains unclear how many people are trapped, National Disaster Management Authority spokesman Anurag Gupta said, adding that both pedestrians and vehicles had been crushed under concrete slabs and metal.

Anil Shekhawat, a spokesman for the National Disaster Response Force (NDRF), said the number of injured stood at 92.

Firefighters and residents were using their bare hands to try to rescue people pinned under a 100-metre-long metal and cement structure that snapped off at one end and came crashing down in a teeming commercial district near Girish Park.

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.

Sorry, this video has expired Rescue workers at scene of collapsed bridge in India

There is hope further survivors will be found, with several policemen buried in their kiosk having had cries for water answered, while rescue teams dig to reach them.

OP Singh, the head of the NDRF, said getting them out alive required caution.

"So far we have located one place where three or four victims are trapped," he said.

"We are supplying water bottles to them and sometime soon we'd like to pull them out alive."

Indian Prime Minister Narendra Modi said his thoughts were with the families of those who lost their lives.

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Collapse highlights lax building standards

Indians have been outraged by K Panduranga Rao, a senior official with the IVRCL group responsible for the project, denying the company was at fault.

"One girder have been missed, then second one has fell. That's how the incident happened. It's nothing but a God's act," he said.

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Architect and urban planner Sudhir Vohra is among many who have begged to differ.

He told Indian channel NDTV India's lax standards and a culture of chasing the cheapest tender were to blame.

"This is not act of God, this is human error, and there is no system of liability," he said.

"We're sitting in 2016, we don't have a system of either controlling the standards of education for design or construction personnel — anywhere. And this is what it comes down to."

'We're leaving — who knows what will happen next'

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 said.

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

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Television footage from the scene showed a bloody hand reaching out from under what appeared to be a massive girder.

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.

"The concrete had been laid last night at this part of the bridge," resident Ramesh Kejriwal said.

"I am lucky as I was planning to go downstairs to have juice. When I was thinking about it, I saw that the bridge had collapsed."

The flyover has been under construction since 2009 and has missed several deadlines for completion.

ABC/wires