A plane has crashed near the Shoreham Airshow in West Sussex and emergency services are at the scene, Sussex Police said.
The aircraft is believed to be a Hawker Hunter, a single-seat jet, and is thought to have come down on the A27, where it hit cars as it crashed.
West Sussex Police said officers along with fire and ambulance services are at the scene dealing with the incident.
Shoreham Airshow also tweeted: “There has been a major incident outside the airfield boundary. The emergency services are responding. Updates will follow when available.”
Onlookers watched in horror as two huge fireballs erupted near a tree line,before plumes of thick, black smoke billowed into the air.
One eyewitness, tweeting as Jon B, said: “It (the plane) turned from the north by Lancing College, went to go low towards the airfield and went behind some trees below where I was.
“Then there was a crunch sound and a ball of flames and thick black smoke came up from the ground.”
West Sussex Police said there were “several casualties” and advised motorists to avoid the A27 for the rest of the day as emergency services deal with the crash.
A spokesman said: “At about 1.20pm the aircraft hit several cars on the A27 just to the north of Shoreham Airport, where an air display is taking place.
“The aircraft ended up in a bush. There have been several casualties but we have no further information on these at the moment.
“The A27 has been closed in both directions and drivers are asked to avoid the area until further notice.”
An Air Accidents Investigation Branch (AAIB) spokesman said: “The AAIB isaware of the incident and we will be deploying a team to investigate.”