Firefighters and residents were trying with their bare hands to rescue those trapped under the wreckage of the metal-and-cement structure that came down near Girish Park in a teeming commercial district.
Television footage from the scene showed a bloody hand reaching out from under what appeared to be a massive girder. People were trying to hand bottles of water to survivors pinned underneath.
"The condition is pathetic. At this moment no one has any clue how many people are trapped," said Raichand Mohta, a police officer at the scene.
A Reuters witness said paramedics had worked to free people from a minibus taxi that was crushed in the collapse.
There was little sign of a coordinated rescue operation, with access for heavy lifting equipment restricted by the proximity of buildings on either side of the flyover and heavy traffic.
The flyover has been under construction since 2009 and has missed several deadlines for completion.
Construction projects in India have long been plagued by lax safety standards.