Rescuers have worked through the night to reach dozens of people believed to be trapped under a collapsed flyover in the Indian city of Kolkata (Calcutta).

The bypass was under construction in the Girish Park area when it collapsed suddenly on Thursday night.

At least 23 people were killed and scores were injured.

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

The cause of the disaster was not immediately clear, but safety issues such as lack of inspections and the use of substandard materials have plagued construction projects in India.

The company in charge of the construction, IVRCL, said it would co-operate with investigators. However one of its senior officials said in a news conference that the collapse had been "an act of god" as the company had a good safety record.

The flyover was in one of Kolkata's most densely populated neighbourhoods, with narrow lanes, and shops and houses built close together, making it difficult to get heavy equipment to the scene.

The BBC's Rahul Tandon in Kolkata described chaotic scenes on Thursday night, with officials trying to clear the area in case the structure collapsed further.

CCTV footage posted on social media appears to show the moment a 100-metre section of the structure collapses, hitting passers-by, auto rickshaws and nearby buildings.

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

Emergency teams are using sniffer dogs, concrete cutters, drilling machines and sensors to detect life, a rescue official told AFP news agency.

A spokesman for the National Disaster Management Authority (NDRF), Anurag Gupta, told AFP that soldiers and NDRF personnel were at the scene alongside hundreds of police and local officials.

The chief minister of West Bengal, Mamata Banerjee, has said the authorities will take "stringent action" against those responsible for the disaster.

A recurring tragedy

• 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