Images posted on social media show a plume of fire and black smoke coming up from the crash site.
All flights to and from the airport have been cancelled while police, emergency services and air accident investigators are attending the incident, London Southend Airport said in a post on X on Sunday.
"We remain on the scene of a serious incident at Southend Airport," the local Essex police said, adding that they were alerted just before 4pm to reports of a collision involving a 12-metre plane.
It was unclear how many people were on the plane.
Images from British newspaper websites, not verified by Reuters, showed a fireball rising into the air above Southend Airport, around 56km east of London.
The East of England Ambulance Service said it had sent four ambulances and other response vehicles.
The airport's website showed five international flights had been cancelled following the accident.
The pilots of the plane waved at children shortly before the collision, a witness said.
John Johnson, who was at the airport with his children and wife, said he saw a "big fireball" after the plane crashed "head first into the ground".
"We all waved at the pilots, and they all waved back at us," he told the PA news agency.
"The aircraft then turned 180 degrees to face its take-off, departure, powered up, rolled down the runway.
"It took off and about three or four seconds after taking off, it started to bank heavily to its left, and then within a few seconds of that happening, it more or less inverted and crashed just head first into the ground," he said.
"There was a big fireball. Obviously, everybody was in shock in terms of witnessing it.
"All the kids saw it and the families saw it," he said.
with PA
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