More are feared trapped under the concrete and steel bridge, and rescue efforts are continuing all night.

Images show residents using their bare hands to help find victims.

Safety issues such as lack of inspections and the use of substandard materials have plagued construction projects in the country.

The accident took place in an area near Girish Park, one of Kolkata's most densely populated neighbourhoods, with narrow lanes, and shops and houses built close together.

The 2km-long (1.2 mile) flyover had been under construction since 2009 and missed several deadlines for completion.

The causes of the disaster were not immediately clear but the company in charge of the construction, IVRCL, said it would cooperate with investigators.

CCTV footage posted on social media appears to show the moment the structure collapses, hitting passers-by, auto rickshaws and nearby buildings.

Witnesses say other cars, buses and lorries have also been hit. People are said to have been living in makeshift homes under the flyover.

The army and national disaster agency in India have joined the rescue operation, which has been hampered by difficulties in getting ambulances and other vehicles close to the site.

Emergency teams have been sent with sniffer dogs, concrete cutters, drilling machines and sensors to detect life, a rescue official told AFP news agency.

(BBC News)