Emergency workers in Kolkata continued to search for survivors underneath a collapsed flyover in a congested area of the eastern city, as the construction firm building the flyover was being investigated.

At least 23 people were killed and more than a dozen were still trapped under the rubble of the under-construction overpass that collapsed in the busy Burrabazar area on Thursday.

Rescuers were using sniffer dogs and special cameras to find trapped people and used saws, small cranes, and their bare hands to dig through the wreckage in search of victims.

Smashed yellow taxis, destroyed rickshaws and the bloody legs of trapped people jutted from the collapsed girders and concrete slabs.

"The condition is pathetic. At this moment no one has any clue how many people are trapped," said Raichand Mohta, a police officer at the scene.