Huge plumes of black smoke are billowing above St George's junior school in Weybridge, Surrey, after an explosion.
Ambulances, a helicopter and fire engines are all at the scene, but no injuries have been reported.
Chris Marchington was in his back garden with his son when he heard a bang, Get Surrey said.
All children and staff are safe at #weybridge fire; Brendan (@Brendan_Surrey) March 13, 2015
"It was the beginning of a smallish fire at that point, on the roof of the new build that's happening at St George's School," he said.
"It's now a major fire."
Fire at St George's school weybridge pic.twitter.com/dKOp9X9zZf; Jamie Millar (@JamieMillarZZ) March 13, 2015
Mr Marchington also said that he heard "very nasty bangs" and described it as "really grim" with a "really nasty smell".
He said the fire appeared to have burnt away all the work done on the site over the past few months.
He also said that the end of Jessamy Road, close to the school, was sealed off, and ash and flecks of charcoal was still coming down over Thames Street and Jessamy Road.
Other residents reported seeing the huge clouds of black smoke above the town.
Neil Davison, general manager of Weybridge Heath Club in Walton Lane near the site, told the newspaper the fire was "absolutely huge".
"There was a small explosion, big enough for us to hear and there were flames on the roof of the new building," he said.
"Things then just escalated really quickly.
A Surrey Fire & Rescue Spokesman said: “We were called to a fire in Thames Street, Weybridge at 12:37 and have sent eight fire engines, a number of specialist vehicles and around 40 firefighters to the scene.
“At this time it is too early to comment further on the incident but updates will be provided on this site when available.”
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