One person has died, three people are missing and five hospitalised in a massive "explosion" at Didcot power station yesterday.. Terrified witnesses described hearing a huge blast which destroyed part of the Oxfordshire plant and shook neighbouring buildings. Emergency services have declared a "major incident" following the blast at about 4pm. The casualties are thought to have been workers preparing two boilers for demolition when part of the plant suddenly collapsed. Mat Carlile, Oxfordshire fire and rescue service area manager, said: "I can confirm search operations are in progress and that there has been one fatality, five persons have been taken to hospital and 3 persons are currently missing." The power station closed three years ago and three of its cooling towers had already been demolished. Up to 200 people still worked at the decommissioned former coal and gas power station. A terrified witness wrote on Facebook: "I'd just left work and was walking home along the perimeter road when there was an almighty rumble followed by dust cloud. "Thought they'd brought down one of the remaining cooling towers at first but all 3 are still there... some other building collapse, I suspect: thoughts to any and all involved!" Another added: "I also heard a humongous rumble. A few of us looked to the sky wondering what the hell it was." The demolition of the site was expected to be finished by the end of the year but it is not yet clear if the explosion was planned. Rodney Rose, the 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. "The fire service is there now and we are still trying to find out if this was a demolition." "At the moment this is being treated as a collapsed building, not an explosion, but there was a bang." A GMB union official told the Press Association: "We understand that workers were preparing two boilers for demolition in the coming weeks. "This led to the collapse of a building." Photographs from the scene showed part of the former coal-fired Didcot A collapsed. The building closed down in March 2013 after more than four decades. David Cooke, whose company Thames Cryogenics have 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. "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."