Nottinghamshire Fire and Rescue Service said the fire started in a public toilet at the station and spread to the roof.
Twelve fire crews were sent to tackle the blaze, which broke out at about 6.30am.
While the fire is still being fought, it has been brought under control and an investigation is under way.
Superintendent Sandra England, who is leading the British Transport Police response, said: "A joint investigation is now under way with Nottingham Fire and Rescue Service to establish the cause of the fire. We would like to hear from anyone who has any information."
She said officers were "grateful for the support we have received from the people of Nottingham" following the incident.
The blaze has caused major disruption, with the station closed and train services expected to be cancelled or delayed for the entire day.
In a video posted on Twitter, incident commander Bryn Coleman said crews had been "offensively firefighting" using breathing apparatus, with firefighters in "some real severe conditions in terms of heat and smoke".
Mr Coleman added that the flames were in the walkways between new and old sections of the station.
Speaking later to Sky News, he said the main area of operation was an "old bridge that's going across platforms six and seven", with fires still burning "in that void area".
There is "smoke damage to the original concourse area of Nottingham station and also smoke damage to the new-build section", Mr Coleman said.
The fire service said no one is believed to have been injured.
Fire officer John Mills said it was the worst blaze local firefighters had seen "for 20 years", the Nottingham Post reported.
"This is about complexity rather than the size of the fire," he said. "People driving through the main roads in the city will barely see any smoke.
"That is because the fire is burning continuously through voids, which puts it out of sight and makes it incredibly difficult for us to locate."
East Midlands Trains said it expected services "to be disrupted on all routes through Nottingham for the whole of Friday 12 January".
Source : https://news.sky.com/story/large-fire-breaks-out-at-nottingham-railway-station-11204147915