The collapse of the Kolkata flyover that killed at least 18 people was an "act of God", according to the contractor.
More than 100 people are feared trapped under the rubble, many with serious injuries.
The flyover, which has been under construction since 2009, collapsed earlier today. The 110-yard section of the raised roadway, designed to help tackle the city's traffic jams, came down in a teeming commercial district near Girish Park.
"It is nothing but God's act. This has not happened to us in 27 years," said K Panduranga Rao, a senior official of the Hyderabad-based IVRCL group.
Video footage aired on TV channels showed a street scene with two auto rickshaws and a crowd of people suddenly obliterated by a mass of falling concrete that narrowly missed cars crawling in a traffic jam.
The army and national disaster agency have joined the rescue operation, which has been hampered by difficulties in getting ambulances and other vehicles close to the site.
India's prime minister Narendra Modi, who is visiting the United States, expressed his condolences to the victims.
Chief Minister Mamata Banerjee, whose centre-left party is seeking re-election in the state of West Bengal next month, rushed to the scene.
"We will take every action to save lives of those trapped beneath the collapsed flyover. Rescue is our top priority," she said.
Banerjee, 61, said those responsible for the disaster would not be spared. Yet she herself faces questions about a construction project that has been plagued by delays and safety fears.
A newspaper reported last November that Banerjee wanted the flyover – already five years overdue – to be completed by February and there were accusations the building had been rushed.
Eyewitness Ravindra Kumar Gupta, a grocer, said two buses carrying more than 100 passengers were trapped. Eight taxis and six auto rickshaws were partly visible in the wreckage.
"Every night, hundreds of labourers would build the flyover and they would cook and sleep near the site by day," said Gupta, who together with friends pulled out six bodies.
"The government wanted to complete the flyover before the elections and the labourers were working on a tight deadline... Maybe the hasty construction led to the collapse."
Additional reporting by Reuters.