Nottinghamshire Fire and Rescue Service tweeted that the flames had "spread to the roof". Crews are "working to stop the fire from spreading".
Ten fire engines are at the building, which has been evacuated. No injuries are reported.
Police said transport would be "severely disrupted", and that Station Street, Carrington Street and Queen's Road had been closed.
East Midlands Trains said it expected "trains to be disrupted on all routes through Nottingham for the whole of Friday 12th January".
"We have a clinical manager and specialist paramedics at the scene," a spokeswoman for East Midlands Ambulance Service said.
Radio reporter Alex Hulse told Sky News that the cause of the fire was still uncertain. There were "no trains, no trams" running, he said.
More follows...