The blast occurred at around 10:30am at the Celsa Manufacturing plant in Splott, Cardiff. Emergency crews were called to the site after eyewitnesses reported hearing a large bang.
Workers told Wales Online the explosion was "very loud - deafening".
It is thought to have been an industrial incident in the basement of the building. At least eight ambulances were on the scene, and Cardiff and Vale University Health Board has implemented its major incidents protoco.
As a result, people have been asked to stay away from the A&E department for the time being.
The secretary of state for Wales Stephen Crabb said: "Our thoughts are with the workers caught up in the reported explosion at the Celsa plant, their families and the emergency services at the scene.
"We are still learning the full extent of what has happened. But in the meantime the Wales Office has been in touch with Celsa to offer any help we can.
"Celsa is a key part of south Wales’ industrial landscape and I have visited the plant on a number of occasions, most recently in September. I am receiving regular updates from the emergency services and I hope there will be a safe recovery for all involved."
More to follow...