One person has been killed, three people are missing and four are in hospital after a building collapsed at Didcot Power Station.
Witnesses described hearing what sounded like an explosion before a huge cloud of smoke spewed above the site in south Oxfordshire.
Emergency services declared a major incident after being called to the scene just after 4pm.
Thames Valley Fire Control Service area manager Mat Carlile said a search was under way for the three missing people.
The collapse happened at Didcot A, a disused oil and gas plant which is currently in the process of being demolished.
The building that came down was meant to be taken down in the coming months.
It remained unclear why it collapsed.
Ed Vaizey, Conservative MP for Didcot and Wantage, told Sky News that it appeared the incident happened while workers were preparing the building for the demolition.
“My understanding is there was no explosion, controlled or otherwise. Obviously we can rule out a terrorist incident. It”s a very secure site,” he said.
A GMB union official also 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.
Specialist police search units, six ambulances, two air ambulance helicopters, fire engines and a hazardous area response team were at the scene.
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”.
There had been initial fears about hazardous material at the site. But it was later confirmed there all hazardous materials had been removed some time ago.
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.”
Didcot B, a natural gas plant which provides power to the National Grid, is still functioning on the site and has not been affected by the collapse at Didcot A.
A National Grid spokeswoman said: “The explosion at the old Didcot power station has had no effect on operation of the National Grid and there has been no interruption to supplies.”
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.