One person has died and three people are missing following the collapse of a building at Didcot A Power Station.
A major incident was declared at the site in south Oxfordshire after initial reports of an explosion at 16:00 GMT.
Thames Valley Fire Control Service confirmed the fatality and also reported four people were injured in a "very severe incident".
The decommissioned Didcot A plant closed in 2013 and demolition work has been taking place.
An Npower spokeswoman said: "We can confirm that shortly after 16:00 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 that it was "working with all stakeholders to establish facts" and it urged concerned relatives of employees to get in touch.
Oxfordshire's Chief Fire Officer, Dave Etheridge, expressed "absolute sympathy and deep thoughts to all the families involved".
He said: "We are doing our very best to secure the return of loved ones as soon as we can."
Deputy Chief Fire Officer Nathan Travis initially said four people had been taken to John Radcliffe Hospital in Oxford, two with serious injuries and two with minor. The number being treated at the hospital was later confirmed by the fire service as five.
A further 50 people were treated at the scene for dust inhalation.
At the scene: Peter Cooke, BBC South Today
Didcot A stands quiet under a full moon, with the only noise the loud hum of generators powering large lights being used by search and rescue teams still slowly sifting their way through a 30ft deep pile of rubble.
Their work is slow and made all the more difficult by the near freezing temperatures. Many have been here since this incident started at 16:00 - and the scale of the collapse is likely to see rescue teams here for some days to come.
Residents living nearby have become well used to buildings being demolished on the site since it was decommissioned in 2013 but these events were unexpected and have caused concern about the future safety of Didcot A.
A 100m cordon is in place around the site and specialist search and rescue teams with dogs from Buckinghamshire, Hampshire and the West Midlands are searching a rubble pile that measures up to 30ft in height.
Mr Travis said the search would be "considerable due to the instability of the site".
"We expect the search to continue throughout the night and possibly into the coming days," he said.
A Health and Safety Executive investigation has also been launched.
Five fire engines were sent to the site along with extensive resources from Thames Valley Police and South Central Ambulance Service.
A joint statement by the emergency services said the building, which is 300m long and 10 storeys high, was due to be demolished in the coming weeks.
A senior spokesman for Npower said that it was a partial collapse, not an explosion, that caused a section of the building to come down.
About Didcot A Power Station
Oxfordshire's coal-fired Didcot A Power Station was turned off in 2013, after 43 years in service.
The site was able to generate 2,000 MW of electricity - enough to meet the needs of two million households - owners RWE Npower said.
The station included six cooling towers, measuring 375ft (114m) in height, which dominated the skyline of the town.
Hundreds of people gathered to watch when three of the towers were demolished in the early hours of 27 July 2014.
RWE Npower expects complete deconstruction of the site by the end of this year.
A gas-burning power station - known as Didcot B - opened in 1997 on the site and continues to operate.
A major fire was declared at Didcot B in October 2014, with 20 fire crews sent to tackle the blaze at its peak. The cause was later confirmed as an electrical fault.
The fire service said dust from the collapse covered a "considerable area" but there were no hazardous materials in the building.
Residents have been urged to remain inside and keep doors and windows closed.
Twitter users described an "explosion" as well as "loads of dust".
Thames Cryogenics tweeted: "Big collapse just happened at Didcot Power Station. Loads of dust."
Adrian Redhead, who lives about a quarter of a mile away, said he had just got home from work after 16:00 when he heard a "massive noise".
He said: "It sounded like a train had come off the rails. Sirens were all over the place. I looked outside and saw all the dust. There were loads of emergency vehicles.
"There are two helicopters above now. A load of dust came over the house."