Avon Fire and Rescue Service said they were called to reports of the blast on Kings Weston Lane, Avonmouth.
A fire service spokesman said: "We have been called to a large explosion at 11.22 in a warehouse in Avonmouth on Kings Weston Lane.
"Crews from Avonmouth, Southmead, Yate, Patchway, Kingswood and Temple are currently attending. We can confirm there are multiple casualties on site.
“The incident is ongoing."
Witness Jawad Burhan took a photo showing a tank that had exploded after the blast.
He told the PA news agency there was a "helicopter looking for missing people" and police had closed a nearby road leading up to the building, which he believed was a waste centre.
"I heard the sound, I'm working beside the building in another warehouse. After ten minutes I saw the helicopter coming and the police," he said.
Reporting by PA