Four people have died and another has been injured in 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.
Three of the people who died worked for the firm, and the other was a contractor. The injured person's condition is not life-threatening.
The blast happened in a silo containing treated biosolids and was not terror related, police said.
A witness reported hearing a "very loud explosion" that "shook buildings", and another said they saw about 10 ambulances driving to the scene.
Police declared a major incident and are investigating the circumstances of the blast.
Ch Insp Mark Runacres, from Avon and Somerset Police, said the explosion happened in a chemical tank at a water recycling centre.
"The fire service led the rescue operation but sadly, despite the best efforts of all those involved, there were four fatalities.
"This is a tragic incident and our thoughts and sympathies go out to them."
The families of those who died have been contacted.
Luke Gazzard from Avon Fire and Rescue Service said the four people died at the scene and there was no report of a fire.
He said emergency services had dealt with "a very, very challenging incident".
Prime Minister Boris Johnson said in a tweet he was "deeply saddened" to learn of the loss of life in the explosion.
"Our hearts go out to the victims and their families. Thank you to the emergency services who attended the scene," he said.
Colin Skellett, chief executive of Wessex Water, said the company's "thoughts are with the families, friends and colleagues of those involved".
He said they were "absolutely devastated that the tragic incident at our site earlier today has resulted in four fatalities".
The company is working with the Health and Safety Executive as part of the investigation.
The silo holds treated biosolids before it is recycled as an organic soil conditioner, Ch Insp Runacres said.
He said a "thorough investigation involving a number of agencies" would be carried out.
"I can reassure people living in the nearby area that there is not believed to be any ongoing public safety concerns."
Biosolids are "treated sludge", a by-product of the sewage treatment process.
According to Wessex Water, the sludge is treated in anaerobic digesters, oxygen-free tanks, to produce agricultural fertiliser and renewable energy.
People have been urged to avoid the area.
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."
'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," 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.
Lorry driver Ronan Doyle said he was parked off Kings 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.
"I continued into the recycling centre and we have just locked ourselves in purely because our way out is blocked."
'Boom and echo'
Sean Nolan, who witnessed the aftermath of the explosion, said he initially thought the noise was from a crash.
"I heard what I thought may have been two trucks colliding by the way it shook the ground... it was big.
"It was quite short-lived, I'd say about two or three seconds. Sort of a boom and echo and then it just went quiet.
"That was it. There was no smoke, there was no after-effects of it."
Labour leader Sir Keir Starmer gave his condolences on Twitter, writing: "My thoughts are with all those who tragically lost their lives today in Avonmouth. My heart goes out to their friends and family."
Darren Jones, MP for Bristol North West, said: "My family and I are keeping those affected in our thoughts and prayers, following the tragic consequences of the explosion in Avonmouth."
He was "pleased that the situation has been contained and that there is no further risk to local people".
Bristol Mayor Marvin Rees said: "This has already been such a challenging year, and this news of further loss of life is another terrible blow.
"As a city we will mourn for them."
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