At least 10 people have been killed and dozens trapped after an under-construction flyover collapsed in eastern Indian city of Kolkata on Thursday afternoon.

People and vehicles on a busy road came under the steel and concrete rubble of the collapsed flyover as it came crashing down in a densely populated neighborhood in Kolkata.

Unconfirmed media reports said at least 150 people were trapped under the rubble.

The collapsed bridge has been under construction since 2009 and it missed several deadlines for completion, Refuters reported.

More than one bus full of passengers was trapped under the flyover rescue forces said.

"We heard a noise and got scared. Yesterday (the bridge) was welded. The bridge collapsed. I think at least 150 people are under the debris," an eye-witness told ANI.