KOLKATA, India — A long section of a road overpass under construction collapsed Thursday in a crowded Kolkata neighborhood, with tons of concrete and steel slamming into midday traffic, leaving at least 21 people dead and scores of others injured in the Indian city, police said.

Rescuers used saws, small cranes and their bare hands to dig through the wreckage in search of survivors.

At least 21 people were killed, a police official said on condition of anonymity because he was not authorized to speak to the media.