A major incident has been declared following an explosion at Didcot Power Station in Oxfordshire.
South Central Ambulance Service said it has sent its hazardous areas response team to the site, as well as three ambulances and the air ambulance.
A spokesman, who said he thought there would be casualties, said: "We are describing it as a major incident."
Thames Valley Police confirmed officers are at the scene but have not released further details.
Npower said it is investigating.
Twitter users have described an "explosion" as well as "loads of dust".
Adrian Redhead said: "All kicking off at Didcot powerstation, explosion, loads of dust, loads of emergency vehicles. Hope everyone is ok."
He also posted a picture of the scene, adding: "Not sure the side of that building should be missing."
Thames Cryogenics tweeted: "Big collapse just happened at Didcot Power Station. Loads of dust. Not sure if it was planned."
South Central Ambulance Service said it has sent its hazardous areas response team to the site, as well as three ambulances and the air ambulance
Three enormous cooling towers at the disused Didcot A Power Station were demolished at the site in 2014.
The coal-fired power station ceased generating in 2013.
There was a huge fire at the site of Didcot B in 2014 due to an electrical fault.
Thames Valley Police said officers were at the scene along with other emergency services