At least three people were killed and dozens injured when a portion of an overpass under construction collapsed in a congested area in the eastern Indian city of Kolkata on Thursday, officials said.

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 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."

The injured were admitted to two hospitals in Kolkata, the capital of West Bengal state, hospital officials said.

Troops of the National Disaster Response Force rushed to join rescue efforts in Bara Bazaar, a busy residential and shopping area in central Kolkata.

O.P. Singh, the NDRF chief, said the operation was a "very, very challenging task."

Building collapses are common in India, where builders use poor enforcement of regulations and use substandard materials.

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