Calcutta: Latest reports say at least 10 people are confirmed to have died, with many more fatalities feared.
About 150 people, most of them construction workers, are thought to be trapped under the debris. Efforts are under way to rescue those trapped while some workers have been pulled out from under large chunks of concrete and metal.
Witnesses say rescuers are using their bare hands to try to save those caught under the rubble.
“We heard a loud rumble and then saw a lot of smoke,” a nearby resident told NDTV.
People are passing water bottles through to those who remain trapped. Construction of the flyover was supposed to be completed three years ago, but it has been delayed several times.
Building collapses are common in India, because of the poor enforcement of regulations and the use of inferior materials.
The National Disaster Response Force (NDRF) has been sent to help rescue efforts.
An NDRF official said: “Our team is on their way to the site with all required equipment, we are also pressing into service canines which will help find trapped people.”