Four people were killed when a newly erected pedestrian bridge over an eight-lane highway collapsed at Florida International University shortly before 2pm on Thursday, Miami-Dade County Fire Chief Dave Downey said.

Eight vehicles were trapped in the wreckage of the 950-tonne bridge and eight people have been taken to hospitals, officials told a news conference.

Emergency personnel with sniffer dogs searched for signs of life amid the wreckage of concrete and twisted metal that rained from the collapsed structure and crushed vehicles on one of the busiest roads in South Florida. It is not known if any survivors are still trapped under the rubble.

Television footage showed firefighters were walking across the flattened wreckage and medics were treating injured people on the road. Emergency personnel appeared to be trying to work their way through a hole in the top of the bridge.