A series of explosions heard across Derby were caused by gas cylinders in a factory fire, senior fire officers have said.
The bangs were heard shortly before 14:00 BST and plumes of smoke were visible for miles.
The blaze, at a rubber safety matting firm, is not thought to be suspicious and was under control by about 16:00.
Train services will be disrupted for the rest of the day and nearby residents were advised to stay indoors.
No injuries have been reported.
Road closures were in place at Pride Park Way and Stores Road to the junction with the Vauxhall garage, and Mansfield Road to the junction with Alfreton Road and St Mary's Wharf police station was evacuated.
Twitter user @martynlocker captured the sound of one of the explosions on his mobile phone.
Derbyshire Fire and Rescue Service said the chemicals involved in the fire made it particularly challenging and were likely to burn for some time.
An investigation into the cause has begun.