Thames Valley Police confirmed emergency services have rushed to the scene after reports of an explosion at around 4pm.
South Central Ambulance Service has declared a “major incident” and confirmed there could be several casualties.
An air ambulance, three road ambulances, a rapid response vehicle and a hazardous area response team have been sent to the scene.
Witnesses described hearing a loud bang before parts of the building collapsed.
Another reported “loads of dust” and “loads of emergency vehicles” at the scene of the explosion.
A police statement added: “Thames Valley Police officers are currently in attendance at Didcot Power Station dealing with an incident with other emergency services.
“Further information will be released in due course.”
Photographs from the scene showed part of a building missing, which appears to be the former coal-fired Didcot A, which closed in 2013.
The power station was earmarked for demolition but a spokeswoman for RWE NPower, which manages the site, said they were not aware of a planned demolition at the time of the explosion or any in the near future.
More to follow..