One person has died and a major search operation is under way for three others after a building collapsed at a power station.
Five people are being treated in hospital after the concrete and steel structure at the derelict Didcot A site in south Oxfordshire came down while it was being prepared for demolition.
A further 50 people were treated for dust inhalation at the scene, where a "major incident" was declared shortly after emergency services were called at 4pm.
Oxfordshire deputy chief fire officer Nathan Travis said specialist urban search and rescue teams, including sniffer dogs, are at the "challenging" site from as far away as the West Midlands.
He said: "The building is potentially 10 storeys high, half of that building has collapsed, so you have got a rubble pile which is approximately 20 to 30 feet deep at the moment.
"The search will be considerable due to the instability of the site, we expect the search to continue throughout the night and possibly into the coming days."
Asked what the chances were of finding the missing alive he replied: "At the moment I can't give you any details on that but it is a substantial collapse of a building."
Pictures from the scene showed a significant chunk of a building in the defunct Didcot A site has collapsed, with a large amount of debris on the ground.
A GMB union official told the Press Association: "We understand that workers were preparing two boilers for demolition in the coming weeks. This led to the collapse of a building."
Oxford University Hospitals NHS Foundation Trust said five casualties were taken to the John Radcliffe Hospital in Oxford. A spokesman said: "Four of the men are in a stable and non-life threatening condition, while one man is in a serious but not life-threatening condition."
There is currently a 100-metre cordon in place and a number of fire engines and an incident support unit have since left the scene.
Residents in a mobile home park close to the site said they heard a rumble and not a bang when the collapse happened.
Colin King, 44, who has lived near the power station for the past 18 years, said: "There was no bang, just a loud rumble. Normally we get given information on anything big they're doing. We usually hear small bangs but I knew something was out of the ordinary.
"Once I heard all the sirens I knew it was serious. Then I saw half the building hanging off - that was a giveaway."
He said the dust from the collapse was the same as when the towers were brought down last year.
Another resident, who did not want to give her name, also said there was no bang, describing the noise as more of a "bubbling".
She told the Press Association: "We did not think anything - my friend and I just looked at each other and thought, what was that?
"From the explosion from the towers coming down last year you could feel it in the floor, but we did not feel anything today.
"It is quite a shock what's happened."
David Cooke, whose company Thames Cryogenics has a building overlooking the power station, said: "Our building shook and as we looked out of the window, the end of the main turbine hall collapsed in a huge pile of dust.
"It totally obscured the towers and must have drifted across the roads and main rail line. What's left looks a tangled mess.
"The dust was hanging over the area for five to 10 minutes.
"First thought was it didn't look planned, followed by the thought that people are going to have been hurt."
Didcot A opened in 1970 as a coal-fired power station and was later converted so it could also generate power from natural gas.
It ceased generation in March 2013 and hundreds gathered to watch when three of its enormous cooling towers were blown up in July 2014 after dominating the town's skyline for more than four decades.
According to the RWE npower website the total demolition of the site is expected to be complete by the end of this year.
The incident comes 16 months after a major fire struck a cooling tower at Didcot B in October 2014.
The blaze affected 50% of the station output - supplying a million homes.
An npwer spokeswoman said: "We can confirm that shortly after 4pm this afternoon part of the boiler house at our former Didcot A power station site in Oxfordshire collapsed while an external demolition contractor was working in it.
"Our thoughts are with the families of all those involved in this tragedy."
Coleman and Company, the firm behind the demolition, tweeted: "We are aware of an incident at Didcot A Power Station. We are working with all stakeholders to establish facts and will keep you updated."