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."
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.
More about:
-1780923278.jpg&h=190&w=280&zc=1&q=100)
