A flyover under construction in the eastern Indian city of Kolkata (Calcutta) has collapsed, killing at least 18 people and injuring others.

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

Horrified by the CCTV footage as #Kolkata #flyover comes crashing down in a crowded area https://t.co/bJN22OQWS1https://t.co/lEi4RXFZTV

— Rita Banerji (@Rita_Banerji) March 31, 2016

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 has 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. But hours later, efforts are still being hampered by difficult access to the scene by ambulances and other vehicles.

At least 70 injured have been taken to hospitals, officials say.

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."

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, told the news agency that two buses carrying more than 100 passengers had been trapped.

"Every night, hundreds of labourers would build the flyover and they would cook and sleep near the site by day," said Gupta, who together with friends pulled out six bodies.

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.