Reports say large concrete slabs fell onto traffic below
At least 14 people have been killed and 150 are feared trapped after a partially built flyover collapsed in the Indian city of Kolkata, according to police.
Witnesses say rescuers are using their bare hands to try to save those caught under the rubble.
One witness has told the New Delhi Television news channel (NDTV): "We heard a loud rumble and then saw a lot of dust in the sky".
Another added: "The area was very, very crowded. Motorized rickshaws, taxis...there was a lot of traffic".
According to local media reports, large concrete slabs from the construction site fell onto traffic below.
Heavy duty cranes have reportedly been brought in to try to move the wreckage, but attempts have so far been unsuccessful.
Television footage from the site shows people passing water bottles through to those who are crying out for help from beneath the debris.
Senior police officer Akhilesh Chaturvedi said 15 critically injured people had been rescued.
"Most were bleeding profusely. The problem is that nobody is able to drive an ambulance to the spot," he added.
Some witnesses have been critical of the initial response, with one saying there appears to be very little coordination on the ground.
Army officers and the National Disaster Response Force (NDRF) have been called in to help rescue efforts.
An NDRF official said: "Our team is on their way to the site with all required equipment, we are also pressing into service canines which will help find trapped people.
"We will start the operations as soon as possible".
India's Home Affairs Minister Rajnath Singh has offered his condolences to the families of those who have died.
"(I am) deeply saddened to know that precious lives have been lost in the Kolkata accident".
Construction of the 2km flyover was supposed to be completed three years ago, but it has been delayed several times.
The region's Chief Minister Mamata Banerjee, who is seeking re-election next month, is already facing tough questions over who is responsible for the tragedy.
A newspaper reported last year that she wanted the project finished by February, although project engineers expressed concern over whether this would be possible.
Building collapses are common in India, partly because of the poor enforcement of regulations and the use substandard materials.