Some of the workers were waving what appeared to be towels and holding on to the scaffolding.
A man was seen scrambling to climb down the building from its outer side while others appeared to be trying to climb to the roof.
Ambulances and fire trucks had raced to the scene and firefighters were putting out the blaze as hundreds of workers and onlookers watched the drama from the street below.
A helicopter was hovering over the building, trying to land on the roof but not succeeding. There were no immediate reports of death or injury.