"Multiple casualties" have been reported after a large explosion at a waste water treatment works.
Firefighters were called to Wessex Water's premises in Avonmouth, Bristol, at about 11:20 GMT.
Emergency services are leading a rescue operation. The blast is thought to have involved a chemical tank, police said.
A witness reported hearing a "very loud explosion" that "shook buildings", and another said they saw "around 10 ambulances driving to the scene".
Police have declared a major incident and are investigating the circumstances of the blast.
Avon Fire and Rescue Service said there were multiple casualties, although it could not say how many.
Avon and Somerset Police said: "An explosion is believed to have occurred involving one of the chemical tanks at Wessex Water's Bristol water recycling centre.
"A rescue operation was subsequently led by the fire service, involving five appliances and two turntable crews and we can confirm there have been a number of casualties."
Jawad Burhan, who took a photo appearing to show a tank that had exploded, said there was a "helicopter looking for missing people".
"I heard the sound, I'm working beside the building in another warehouse.
"After 10 minutes I saw the helicopter coming and the police," he added.
Ch Insp Mark Runacres said inquiries were taking place into "exactly what happened".
"Officers remain at the scene and are likely to be [there] for some time as we work together with other agencies in dealing with this major incident," he said.
"A cordon has been set up and we'd advise members of the public to avoid the area at this time where possible to allow the emergency services to carry out their work."
'Everything was shaking'
Kieran Jenkins, who works nearby, said he was inside a warehouse when he heard a "big bang".
"The whole warehouse was shaking, and we literally stood there in shock because everything was shaking." he said.
"We thought everything was going to fall and we came out and all we could see was people running - it was a bit of a shock, really."
Bristol Waste, which runs the nearby Avonmouth recycling centre, tweeted it had closed the site temporarily.
At the scene: Jon Kay, BBC News
Roads to the area have been blocked off and there are lots of firefighters going on to the site wearing special equipment.
There are also blue police lights flashing and a number of ambulances waiting as the site is searched.
There is a tall metal vertical tank which appears to have been ripped open somehow.
But there is no smoke or anything else still in the air. It's just the remains of the tanker.
Lorry driver Ronan Doyle said he was parked off King's Weston Lane about to enter the recycling plant when he heard the explosion.
"There was a quieter "whoosh" first, followed by a much louder and more intense noise," he said.
"It sounded like someone had driven into the lorry - the noise was so loud it didn't sound like anything I've ever heard before and it was followed by a loud bang."
He added: "I continued into the recycling centre and we have just locked ourselves in purely because our way out is blocked."
'Deeply concerned'
Darren Jones, MP for Bristol North West, said: "This is a serious incident and my thoughts are with those workers who have injured on site today.
"I'm grateful to our fire, police and ambulance staff for their quick response."
Home secretary Priti Patel said in a tweet: "Deeply concerned at reports of a warehouse explosion in Avonmouth.
"My thoughts are with those affected and our brave emergency services working hard to resolve this incident."
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https://news.google.com/__i/rss/rd/articles/CBMiNmh0dHBzOi8vd3d3LmJiYy5jby51ay9uZXdzL3VrLWVuZ2xhbmQtYnJpc3RvbC01NTE3MzY2N9IBOmh0dHBzOi8vd3d3LmJiYy5jby51ay9uZXdzL2FtcC91ay1lbmdsYW5kLWJyaXN0b2wtNTUxNzM2Njc?oc=5
2020-12-03 16:19:00Z
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