Kolkata, March 31,2016:Â Many people are feared trapped under the concrete and steel bridge, which fell on a busy road.
Construction projects in India have often suffered from safety issues with frequent collapses.
Experts say there is a lack of inspections and substandard materials are used.
The flyover has been under construction since 2009 and has missed several deadlines for completion, Reuters news agency said.
Emergency teams have been sent to the area near Girish Park, but witnesses say there is little sign of co-ordinated rescue efforts.
The collapsed structure appears to have hit cars, trucks and nearby shops.
The BBC’s Sanjoy Majumder says the accident took place in one of Kolkata’s most densely populated neighbourhoods, with narrow lanes, and shops and houses built close together.