The flyover collapse took place at Burrabazar, a busy area in central Kolkata near Ganesh Talkies.
The army used heat cameras to track survivors. Challenged by the metal and concrete, rescuers used cutters, drilling machines and sensors.
A 140-tonne crane brought to the scene was unable to lift a huge slab under which many people were feared trapped.
The 2-km flyover was coming up at Burrabazar, one of the oldest and most congested parts of the city.
"We heard a loud sound, like a bomb blast, and then saw a lot of smoke," said a resident.
The bridge has been under construction since 2009 and has missed several deadlines for completion.
Helpline numbers: 1070, 033-2214-3526, 033-2253-5185, 033-2214-5664
Track Latest News Live on NDTV.com and get news updates from India and around the world
