At least 15 people were killed in Kolkata and many more are trapped after a flyover under construction for years in a crowded commercial area came crashing down on Thursday.
Many are feared dead in what the state government called a monumental tragedy.
"I think at least 150 people are under the debris," said a witness.
Prime Minister Narendra Modi said from Washington that he is "shocked and saddened".
Moments after the crash, recorded on CCTV, firefighters and residents tried to pull out those trapped under chunks of concrete and metal. Later, people were seen clapping when around 400 army personnel joined what was turning out to be a colossal rescue effort in the fading light.
Television footage showed a body being pulled out and a bloody hand reaching out from under the rubble.
Visuals also showed cars and a minibus buried under the collapsed bridge. Fires erupted when petrol tanks of cars were crushed.
The army is using heat cameras to track survivors. Challenged by the metal and concrete, rescuers used cutters, drilling machines and sensors.
A 140-tonne crane brought to the scene was unable to lift the concrete slab under which many people were feared trapped.
The 2-km flyover was coming up at Burrabazar, a busy area in central Kolkata near the famous Ganesh Talkies. It has been under construction since 2009 and has missed several deadlines for completion.
"We heard a loud sound, like a bomb blast, and then saw a lot of smoke," said a resident of the teeming area.
Mamata Banerjee cancelled her campaign for the upcoming state polls and went to the site as rival parties attacked her government.
"The guilty will not be spared. Our main focus right now is rescuing those trapped," Ms Banerjee said.
Union Minister Babul Supriyo, one of the two BJP parliamentarians from Bengal, alleged that the flyover was not properly planned.
"It is complete callousness of the state government. The flyover was touching the windows of houses," Mr Supriyo told NDTV.
Hyderabad-based IVRCL group, which was building the flyover, said its collapse was an "act of God".
Kolkata is the venue of this Sunday's World T20 cricket final.