Updates from 23 February 2016
Dave Gilyeat
BBC News
The owner of Didcot A Power Station says it is working with its contractor, which was helping to decommission the site, to establish what has happened.
A caller called Tim, who was working at Southmead Industrial Estate at 16:00 today, told BBC Radio Oxford he saw a "huge plume of dust" come from the power station site.
"There was a huge noise and I caught it collapsing as I looked out of the window. People were definitely shocked and didn't know whether to get back to work.
"When I had a closer look it was a twisted mass of metal."
Oxford University Hospitals NHS Foundation Trust said it is responding to the major incident at Didcot A Power Station, with casualties being directed to the John Radcliffe Hospital.
"Due to a major incident at Didcot please consider carefully before coming to A&E," it added.
Image source, Bill NichollsDavid 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.
Image source, PA"It totally obscured the towers and must have drifted across the roads and main rail line.
"What's left looks a tangled mess. First thought was it didn't looked planned, followed by the thought that people are going to have been hurt."
Amanda Dellor
BBC Radio Oxford
The light's fading here now and the sun's setting but the scene is being lit up by all the blue and red lights flashing around the area and a major investigation is under way.
A significant chunk of a building in the defunct Didcot A site has collapsed, with a large amount of debris on the ground.
The total demolition of the site was expected to be by the end of this year.
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Didcot A Power Station, where the collapse and dust explosion has happened, was in the process of being decommissioned.
Hundreds of people gathered to watch its 375 ft (114m) towers come down in 2014, marking the end of an era for the coal-fired power station, which dominated the skyline around the town since 1970.
Image source, PAYou can view a timeline of the power station's eventful life here.
Didcot B continues to provide power - it provides about 1.3GW of electricity to the grid, enough power for a million homes.
Our reporter at the scene says at least two helicopters are above the power station.
Three police cars and two fire engines arrived on site very quickly.
The dust cleared fairly quickly and there is no signs of any flames, he added.
Blaine Morris-Smith captured footage of the dust cloud following the blast at the power station.
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Bill McKinnon, 59, tells us: "I was sitting in the front room and I heard - I think it was a bang and then this really loud rumble.
"I looked across and thought, 'oh, they're blowing up the power station again'.
Image source, @REALANDYB"A big cloud of dust came through our village like it did a couple of years ago when they blew up the towers.
"Once that had cleared quite a good portion of the main building where they used to keep the generators is now missing."
Adrian Redhead tells us: "There was loads of dust and smoke and half the building's missing.
"Half of it has come down and the rest of it is hanging by a thread."
Image source, Blaine Morris-SmithThis footage was captured in the aftermath of the incident at Didcot A Power Station, as a large dust cloud envelops the site.
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Rodney Rose, deputy leader of Oxfordshire County Council, has told the Oxford Mail, external he has been informed by emergency services of at least one fatality so far.
A major incident has been declared with several casualties after an explosion at Didcot A Power Station, South Central Ambulance Service said.
Police were called to the site at 16:00.
Both and Oxford University Hospitals Trust and RWE npower tell us they're aware of a situation at the Didcot site but are awaiting more details.
Eyewitness Adrian Redhead tells us: "I’d just got home from work just after 4pm when there was a massive noise – it sounded like a train had come off the rails.
Image source, Samuel Green"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.
"I’ve heard there are people inside. My friend’s husband is a paramedic on his way there."
Readers may recall that three enormous cooling towers at the disused Didcot A Power Station were demolished at the site in 2014.
The coal-fired power station ceased generating in 2013.
There was a huge fire at the site of Didcot B in 2014 due to an electrical fault.