One person has been killed, three people are missing and five in hospital after a building collapsed at Didcot Power Station.
A major incident has been declared after reports of a loud explosion at the site in south Oxfordshire around 4pm.
Thames Valley Fire Control Service area manager Mat Carlile said a search was underway.
The collapse happened at Didcot A, a disused oil and gas plant which is currently being demolished.
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.”
Pictures show a part of Didcot A has collapsed, with large chunks of debris scattered on the ground.
Mobile phone footage also showed a huge amount of smoke rising from the station.
People have been advised by the fire service to stay indoors as a huge amount of dust from the collapse had covered “a considerable area”.
No hazardous materials were in the building, the fire service said.
Specialist police search units, six ambulances, two air ambulance helicopters, fire engines and a hazardous area response team were at the scene.
Rodney Rose, deputy leader of Oxfordshire County Council, told the Oxford Mail: “At the moment this is being treated as a collapsed building, not an explosion, but there was a bang.”
A Oxford University Hospitals NHS Foundation Trust spokesman said the casualties were being taken to the John Radcliffe Hospital.
The public has been asked to stay away from the hospital unless suffering “serious or life-threatening emergencies”.
David Cooke, who works in a building next to Didcot 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.”
A natural gas plant, called Didcot B, is still functioning on the site.
Didcot A ceased operation in March 2013 after more than 40 years or generating power for the area.
It was originally a coal-fired station, but was later converted so it could also generate power from natural gas.
A huge fire ripped through Didcot B in October 2014. No one was injured, but the blaze put half of the gas-fuelled plant – which supplies a million homes – out of action.