A major incident has been declared and one person has reportedly been killed after a building collapsed at Didcot Power Station.
It follows reports of a loud explosion at the site in south Oxfordshire.
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."
Rose added: "At the moment this is being treated as a collapsed building, not an explosion, but there was a bang."
It is unclear whether the collapse was part of planned demolition work at the site.
A Hazardous Area Response Team, six ambulances and two air ambulances have been sent to the scene, South Central Ambulance Service said.
"We are describing it as a major incident," a spokesman said, adding that "there might be casualties".
Pictures from the scene show a large part of the former oil and gas plant on the site - called Didcot A - has collapsed.
Didcot A shut in 2013, but a natural gas plant, Didcot B, was still functioning at the site.
Thames Valley Police said it was called to the site at 4 pm local time.
"Officers are at Didcot Power Station at an incident with other emergency services. Further details will be released in due course. #Didcot," the force tweeted.
David Cooke, whose company Thames Cryogenics overlooks the 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.
"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."
It comes 16 months since a major fire struck a cooling tower at Didcot B, another plant on the site.