Photos show one of the buildings at Hoo Farm Industrial Estate, Frederick Road, well alight with a fireball bursting out the top and smoke billowing into the sky.
Jon Laight, Hereford & Worcester Fire and Rescue Service Group Commander, described the fire as "significant".
In a statement on Twitter he said: "This is a significant fire so please avoid the area. For those in Kidderminster, keep your windows and door shut due to the large amounts of smoke being produced.
"Over 20 fire appliances on scene working with on site experts and other emergency services."
Roads going into the industrial estate have been closed, with West Mercia Police asking motorists to avoid the area and seek alternative routes where possible.
Worcestershire County Council says the A449 in Kidderminster is closed between Hoobrook Roundabout and the junction of A442 Droitwich Road.
A statement from Hereford & Worcester Fire and Rescue said: "Hereford & Worcester Fire and Rescue Service are currently attending a large fire on Frederick Road, Hoo Farm Industrial Estate in Kidderminster.
"People are asked to avoid the area and keep doors and windows closed if they are within the area of the smoke plume."
Kidderminster College has also closed its Vocational Skills Centre, although the main campus remains open as usual.
West Midlands Ambulance Service is also supporting fire crews.
The service tweeted: "Due to the large amounts of smoke, all local residents are advised to keep doors and windows shut where possible."