Kolkata bridge collapse: At least 21 dead and 150 feared trapped after flyover collapses

The partially-completed concrete and steel bridge fell on to traffic on a busy road

At least 21 people have been killed and 150 are feared trapped after a bridge collapsed in north Kolkata, India.

The partially-completed concrete and steel bridge fell on to traffic on a busy road. Cars, trucks and rickshaws can be seen under the debris.

Firefighters and residents were trying to rescue those trapped under the debris with their bare hands.

“The condition is pathetic. At this moment no one has any clue how many people are trapped,” Raichand Mohta, a police officer at the scene, told reporters.

Troops from the National Disaster Response Force joined rescue efforts in Bara Bazaar. Some workers have been pulled from under large chunks of metal and concrete.

Huge cranes and other rescue equipment have been sent to the site and rescuers have begun clearing the rubble.

"The area was very, very crowded. Motorized rickshaws, taxis ... there was a lot of traffic," one witness told NDTV:

Another added: "We heard a loud rumble and then saw a lot of dust in the sky."

More than 70 injured were admitted to two hospitals in Kolkata - formerly Calcutta - the capital of West Bangal state.

It is unclear how many people remain missing.

Prime Minister Narendra Modi has expressed his condolences on Twitter.

The construction of the flyover has been delayed six times. It was supposed to have been completed three years ago.

Additional reporting by agencies