David Cooke, whose company Thames Cryogenics has 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.
Four people are feared dead this evening following a major building collapse at Didcot Power Station.
One person is dead, three are missing and five are in hospital following the incident this afternoon.
A doctor is on scene too so rescue teams are looking for signs of life.
There were no explosives, since the building was being prepped for demolition.
The incident took place at about 4pm in the turbine hall of the former coal-fired Didcot A plant in south Oxfordshire, which closed in 2013 and was in the process of being demolished.
It is thought to have been caused by work to to prepare two boilers for demolition.
• One person dead, five injured and three missing at power station • Major incident declared after turbine hall collapses at Didcot A • Parts of site were being prepared for demolition later this year • Witnesses report hearing huge explosion and buildings shaking • Aerial pictures show huge pile of debris at site that closed in 2013 • It comes 16 months after major blaze at Cooling Tower at Didcot B • Didcot A power station was due to be demolished by end of year Four people are feared dead after a building collapse at Didcot Power Station.
Emergency services declared a “major incident” with one person dead, three missing and five in hospital following the collapse.
The collapse comes just over a year after a major fire struck a cooling tower at the Didcot B power station in October 2014.
It began in a cooling tower and later spread to others, with one of the modules used to generate electricity put out of action for a week.
It ceased generation in March 2013 and hundreds gathered to watch when three of its enormous cooling towers were blown up in July 2014 after dominating the town's skyline for more than four decades.