A plane thought to have been involved in an aerial display has crashed at Oulton Park in Cheshire.
The motor racing track in Little Budworth is currently hosting the CarFest event created by TV and radio presenter Chris Evans.
Evans tweeted: "I am very sad to announce there has been a serious air incident at CarFest North off site.
"The emergency services are in attendance."
Eyewitnesses described horrified spectators screaming as a plane nose dived suddenly during a display.
CarFest is described on its Twitter profile as Evans's "dream of the perfect family festival", featuring cars, planes and music. It is raising money for BBC Children In Need.
The event in Cheshire began yesterday and is due to finish tomorrow.
A second event is due to take place at Laverstoke Park Farm in Hampshire between August 28-30.
CarFest organisers said in a statement: "There has been a serious air incident outside the site of CarFest North, Oulton Park, Cheshire, during an air display at around 1400 today.
"The emergency services are in attendance and the air accident services have been notified.
"The event is continuing and more information will be released when it becomes available. No further comment will be made at this time."
Onlooker Johnathan Randall tweeted: "Think I've just witnessed a plane crash? Crowd looks glum, confused, concerned. Everyone on their phones."
Emma McEvoy added on Twitter: "OMG plane crashed right in front of our eyes at Carfest hope the pilot ejected!! Whole crowd went silent!"
Evans is a car enthusiast who has just become the new presenter of Top Gear.
He is taking over from Jeremy Clarkson whose contract was not renewed after he punched a member of the show's staff during a fracas. Former Top Gear co-hosts Richard Hammond and James May have now also teamed up with Clarkson for a motoring show on Amazon.