Main line train services have been suspended and residents are being advised to stay indoors.
According to police, the blaze at a commercial building on Enterprise Way is not thought to be suspicious.
The nearby St Mary's Wharf police station has been evacuated. No injuries have been reported so far, the East Midlands Ambulance Service says.
The smoke subsided after an hour, but the cordon was expanded for "people's safety". Nearby residents have been advised to close their doors and windows.
The cause of the fire is not yet known.