The BBC's Rahul Tandon reports from the "chaotic" scene shortly after the collapse, where people used their bare hands to try and move concrete
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.
The flyover collapsed in a busy area of Kolkata (Image: Debasish Bhaduri)
Volunteers have used their bare hands to help those trapped under the rubble
Scene of flyover collapse (Image: Reuters)
The area is surrounded by tall buildings and heavy traffic (Image: Reuters)
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 - Scene of flyover collapse (Image: AP)
Chaotic scenes: Rahul Tandon, BBC News in Kolkata
These are chaotic scenes, as officials are desperately trying to clear the area, worried that the rest of the flyover may collapse. They are trying to bring in equipment to clear the rubble and rescue those who may be under it. But the proximity of the buildings may limit the access for heavy lifting equipment to the area.
It is a race against time and the death toll may rise. A rescue official said there were buses trapped.
Locals have been digging through the huge slabs of rubble with pick axes while police hold back relatives desperate for news of the missing.
As the operation continues, angry relatives arrive looking for their loved ones. At the same time, more bodies are being taken away.
Another eyewitness, Ravindra Kumar Gupta, together with friends pulled out six bodies.
He told the news agency: "Every night, hundreds of labourers would build the flyover and they would cook and sleep near the site by day."
India's Prime Minister Narendra Modi, who is in Washington in an official visit, tweeted that he was "shocked and saddened" by the collapse.
"My thoughts are with the families of those who lost their lives in Kolkata. May the injured recover at the earliest," he added.
A recurring tragedy
Collapsed building in Chennai in June 2014 Image copyright AP
- August 2015: At least 11 people die after a building collapses near India's western city of Mumbai
- July 2015: Five killed after a four-storey building collapses in Delhi
- April 2013: 74 people die after a high-rise residential building in Mumbai collapses
- June 2014: At least 10 people killed when a three-storey building in Delhi collapses
- June 2014: At least 60 people killed when a building collapses in a Chennai suburb
- November 2010: 69 killed and more than 80 injured in a collapse in Delhi
- September 2009: Chimney of a power plant in Chhattisgarh caves in, killing 40 people
Source: Bbc.com
