One person is dead, three are missing and five are in hospital following a building collapse at Didcot power station, Thames Valley Fire Control Service said.
Emergency services were called at 4pm to the site in south Oxfordshire amid reports of an explosion.
Rodney Rose, deputy leader of Oxfordshire County Council, told the Oxford Mail: 'I have been told there has been one fatality, but the rest is currently unknown. The fire service is there now and we are still trying to find out if this was a demolition.'
Mr Rose, who sits on a committee responsible for Thames Valley Fire Service, added: 'At the moment this is being treated as a collapsed building, not an explosion, but there was a bang.'
Six ambulances and two air ambulance have been sent to the scene, South Central Ambulance Service said.
A spokesman said: 'We are describing it as a major incident,' adding that he thought there would be casualties.
Photographs from the scene showed part of a building missing, which appears to be the former coal-fired Didcot A plant.
The power station was scheduled for demolition later this year but no demolition work was planned for today.
David Cooke, whose company Thames Cryogenics have 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 looked planned, followed by the thought that people are going to have been hurt.'
The plant closed in 2013 and hundreds gathered to watch when three of its enormous cooling towers were demolished.
Oxford University Hospitals NHS Foundation Trust said 'casualties' were being taken to the John Radcliffe Hospital in Oxford and asked the public to stay away unless suffering 'serious or life-threatening emergencies'.
A spokesman said: 'Casualties are being directed to the emergency department of the John Radcliffe Hospital which has been made ready to receive them.
'We are not able to release any information on numbers of casualties at this time.
'We are grateful to members of the public for avoiding attending our emergency department for anything other than serious or life-threatening emergencies.'
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.'
The official said he did not believe there had been an explosion.
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