Unconfirmed sources have put the death toll at 22 after the collapse of the huge girder that crushed people and vehicles which the flyover builder described as an "act of God", drawing flak.
The West Bengal government ordered a high-level probe into the tragedy that triggered a blame game with the opposition accusing the Mamata Banerjee government of callousness.
The police said that 18 persons died in the mishap in the Burrabazar area, which houses the city's largest wholesale market, while several others were injured.
The injured were rushed to the hospitals in the vicinity. Two policemen and one Jorabagan traffic sergeant Sandip Haldar were critically injured.
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State administration officials said 62 people were injured and taken to hospitals, while several others were feared trapped under the debris.
"It is nothing but God's act," claimed Panduranga Rao of the Hyderabad-based IVRCL Construction company engaged in the flyover's construction.
Chief Secretary Basudeb Banerjee said that a high level inquiry has been ordered into the flyover collapse.
A number of vehicles were crushed under the concrete and steel debris.
CCTV footage showed people, cars, autorickshaws and hawkers buried under the collapsed structure, while some trying to flee the spot were also trapped.
The footage also caught a bloodied hand from under a girder gesturing for help as people handed over water bottles to survivors trapped underneath.
Chief Minister Mamata Banerjee, who visited the spot cutting short her election campaign, said, "Stringent action will be taken against officials of the construction company and others involved."
West Bengal Governor K N Tripathi, who visited the spot, sought a report of the incident from the state government.