UPDATE: Temporary bridge on M1 in JHB collapses, 2 dead & at least 20 injured

The scaffolding of a pedestrian bridge under construction in Sandton has collapsed.

JOHANNESBURG - The scaffolding of a pedestrian bridge under construction in Sandton has collapsed.

The bridge was being built parallel to Grayston Drive on the M1 highway.

Terrible accident on M1. Bridge under construction has collapsed. pic.twitter.com/X6q7demKzW — Baba kaAphiwe (@tutpack) October 14, 2015

It's understood the scaffolding fell onto at least one car travelling below. WATCH: A bystander filmed the scene where the scaffolding of a bridge being built parallel to Grayston Drive over the M1 highway, near Sandton, collapsed

Motorists at the scene describe what they saw.

"There is definitely a minibus taxi that was damaged, but it looks like there may be a car underneath the bridge."

A woman says cars began reversing when they were made aware of what happened.

"There were guys just running, plain clothed guys just running towards the cars. I said 'what is going on' and they said 'the bridge has collapsed' and suddenly everyone started reversing."

WATCH: Medics and firefighters working at the scene

A third motorist says he drove under the bridge just as it was collapsing.