At least 23 people were killed after a section of an under-construction overpass collapsed in a crowded area of the eastern Indian city of Kolkata on Thursday.

Dozens were trapped as rescue teams struggled to free survivors.

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.

"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.

Video captures deadly bridge collapse in India

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

Local TV footage 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 the traffic jam.

Witness 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."

video from another angle by Dhiman Hazra, Bridge collapsed in Kolkata, India, dozen dead, over 150 trapped pic.twitter.com/wS28W6J27R — Khalid Khan™ (@khalidkhan787) March 31, 2016

Al Jazeera's Divya Gopalan, reporting from New Delhi, said the accident occurred in a "very busy area.

"Many people are raising questions about how this could have happened. The flyover has been under construction for five years and has missed several deadlines for completion," she said.

"So one of the reasons for the collapse could be lack of quality and safety control."

Indian company IVRCL was building the 2km Vivekananda Road flyover, according to its website, and its director of operations, AGK Murthy, said the company was unsure of the cause of the disaster.

"We did not use any inferior-quality material and we will cooperate with the investigators," Murthy told reporters in Hyderabad, where the firm is based. "We are in a state of shock."

Construction collapses are common in India, where regulations aren't strictly enforced and builders often use substandard materials.