Image courtesy of Twitter. Efforts are underway to rescue those trapped under the debris.

KOLKATA, India (AP) — At least 21 people were killed and dozens injured when a portion of an under-construction flyover collapsed in a congested area in the eastern Indian city of Kolkata on Thursday afternoon.

Efforts are underway to rescue those trapped under the debris. Many cars, trucks and other vehicles lay under the large chunks of concrete and metal debris.

The Indian Army, medical teams and disaster response forces have been called in for the rescue operations. Several reports described scenes of firefighters and onlookers trying to pull out persons trapped under the concrete using their bare hands.

Situated in a densely populated area in north Kolkata, the 2 km-long flyover had been under construction since 2009, but had faced repeated delays and missed several deadlines for completion.

The construction of the flyover had been contracted to a private infrastructure company IVRCL, which has called it an act of god. A company spokesperson said that 45% of the flyover was unfinished and that up to 40 workers were at the site before the collapse.

"The area was very, very crowded. Motorized rickshaws, taxis ... there was a lot of traffic," one witness told the New Delhi Television news channel, or NDTV.

Another added: "We heard a loud rumble and then saw a lot of dust in the sky."

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