An aircraft has crashed at London Southend Airport.
Essex Police said it was alerted to a 12-metre plane on fire at the site in Southend-on-Sea shortly before 16:00 BST on Sunday.
The airport confirmed there had been a "serious incident" and it was working with the local authorities.
John Johnson, 40, from Billericay, Essex, was watching planes with his wife and children when he saw a "huge fireball".
According to the airport's website, four flights scheduled to take off on Sunday afternoon have been cancelled.
The airport added it would provide more information "as soon as possible".
Mr Johnson was at the airport with his wife and children to watch aircraft activity as "my sons are really into planes".
His family waved at the pilots of the aircraft before it went down, he said.
"You could see [the pilots] smiling and we all kind of smiled back, the aircraft then turned 180 degrees to face its take off point, powered up its engines and powered past us, carrying on down the runway," he told the BBC.
"It took off in probably three or four seconds. It started to bank heavily to its left.
"I said to my wife, that's unusual. We don't find aircraft normally turning at that stage in their ascent.
"Within a few seconds of that, the aircraft almost inverted and hit the grounds. There was a huge fireball."
He called 999 to report the incident.
A spokesperson for Essex Police said: "We are working with all emergency services at the scene now and that work will be ongoing for several hours.
"We would please ask the public to avoid this area where possible while this work continues."
Essex County Fire and Rescue Service said it was called to the incident at 15:58 and has asked people to avoid the area if possible.
A spokesperson said: "Crews from Southend (two), Rayleigh Weir and Basildon (two), along with off-road vehicles from Billericay and Chelmsford attended.
"We are continuing to work at the scene with our emergency services and aviation partners."
The Labour MP for Southend West and Leigh, David Burton-Sampson, said on X: "My thoughts are with everyone involved."
Matt Dent, Southend City Council's cabinet member for business, culture, music and tourism, said on X: "I am aware of the live serious incident ongoing at London Southend Airport.
"At present all I know is that a small plane has crashed at the airport. My thoughts are with all those involved, and with the emergency services currently responding to the incident."
