A flyover under construction in the Indian city of Kolkata (Calcutta) has collapsed, killing at least 20 people and injuring nearly 100.

A number of people are feared trapped under the concrete and steel bridge, which fell on a busy road.

Images show residents using their bare hands to help the rescue effort.

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.

Scene of flyover collapseThe flyover collapsed in a busy area of Kolkata
Scene of flyover collapseVolunteers have used their bare hands to help those trapped under the rubble
Scene of flyover collapseThe area is surrounded by tall buildings and heavy traffic

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.

Ramesh Kejriwal, an eyewitness, told Reuters: “There was a loud sound which scared us. The concrete had been laid last night at this part of the bridge.”

Map of Kolkata in India