The accident happened close to the back of the Royal Courts of Justice in the centre of Belfast. The men were working on the second floor of new office block for the Northern Ireland Law Society when the structure collapsed.
Eyewitnesses said the floor gave way as concrete was being poured onto the top of the planned six-storey office block in Victoria Street.
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They said some workers fell about 30ft to the ground and were trapped under the debris when the accident happened on Monday soon after 10.40am.
Nine fire engines and a specialist fire crew and search dog, as well as ambulance crews and police officers attended the scene.
It is believed there may have been up to 60 workmen on the site at the time of the accident.
Five of the workers are in serious but stable conditions and the sixth is understood to have less serious injuries.
The construction company in charge of the project, Turkington Construction, has said it will be working closely with the Health and Safety Executive throughout the investigation.
In a statement, Turkington Construction said it "deeply regrets the incident which took place today (Monday) at Law Society house.
"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."
(VB/JM)
