"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 UK newspaper websites, not verified by Reuters, showed a fireball rising into the air above Southend Airport, about 56km east of London.
The East of England Ambulance Service said it had sent four ambulances and other response vehicles.
The relatively small airport's website showed five international flights had been cancelled following the accident.
Media reports said the plane was a medical transport jet equipped with medical systems for transporting patients, specifically a Beechcraft B200 Super King Air heading to the Netherlands.
John Johnson, who was at Southend Airport with his family at the time of the crash, said he saw a "big fireball" after the plane "crashed head first into the ground".
Johnson told the PA news agency the aircraft "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.
"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. I phoned 999, reported it.
After the crash, Johnson said the airport's fire service responded straight away, with two fire engines sent to the crash site, followed by local police, ambulance and fire services.
Johnson said that before the plane got into position for take-off, he and his family waved at the pilots and "they all waved back at us".
The local member of parliament, David Burton-Sampson, urged people to stay away and let the emergency services do their work.
"My thoughts are with everyone involved," he said.
with PA and AP
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