Concrete was being poured onto the second-storey slab when the collapse occurred. Local engineers said that the falsework for the concrete pour collapsed without warning.
The men fell approximately 10m and are all reported to be in a stable condition. Five remained in hospital as NCE went to press.
It is believed that up to 60 people were working on the site at the time of the accident.
Health and safety officials in Northern Ireland have closed down the site and are carrying out a full investigation.
Turkington Construction was contractor on the building, which was to be the new headquarters for the Law Society of Northern Ireland. In a statement Turkington Construction said it "deeply regrets the incident".
"We are now working closely with the Health and Safety Executive to investigate the causes of the incident and to ensure the site is secured to the public and our workforce."