Seven people have died after a Hawker Hunter jet crashed into several vehicles during Shoreham Airshow.
South East Coast Ambulance Service said the victims died at the scene, while another person is critical in hospital. The number of injured stands at 14.
The plane crashed on the nearby A27. Witnesses said it was performing a loop but could not complete the manoeuvre.
Prime Minister David Cameron sent his "heartfelt condolences" to the families of those who died in the crash.
Eyewitness Stephen Jones said: "The aeroplane involved is a Hawker Hunter T mark 7. And he'd just begun his flying display.
"He'd gone up into a loop and as he was coming out of the loop I just thought, you're too low, you're too low, pull up.
"And he flew straight into the ground either on or very close to the A27, which runs past the airport."
Ailish Southall, who was driving along the A27 in West Sussex with her two children, said the plane came down close to them.
"We were waiting for it to go back up and it didn't - it seemed to kind of split in two," she said.
"There were huge amounts of fire and we ran from the car to kind of avoid the debris because we were about 15 metres away, we were just across the road from where the accident happened."
At the scene
BBC Sussex reporter Simon Jenkins
I'm standing on the middle of the A27 and it's deserted.
The police are taking no chances whatsoever. I've been asked to move back in case of any further explosions.
I've been speaking to people and they are absolutely stunned. The plane seems to have stalled mid manoeuvre and came crashing down on the A27.
The acrid smell and thick black smoke has just been fanning across the area and the skies are completely quiet.
A spokesman for Sussex Police said emergency services had declared a "major incident" with a number of helicopters deployed to the scene following the crash which happened at about 13:20 BST.
He said: "The casualties are all believed to have occurred on the road at this time and there are not thought to be any injuries to anyone actually on the airfield.
"The Air Accident Investigation Bureau are attending the scene and have asked anyone who has video or photographs of the plane in the time leading up to and including the crash to hold on to them and await advice as to whether they may be wanted to assist with the investigation."
Shoreham resident Dave Penwarden, 51, said he saw the plane explode.
"The plane was coming out of a loop-the-loop, it was fairly low," he said.
"It just didn't seem to have enough speed to come out of the loop, instead of powering out it dropped too fast and hit the ground.
"There was a massive fireball and an awful lot of smoke. Afterwards there was a stunned silence."
Gairo Gomez, who works nearby, said: "I heard a huge bang and the glass was shaking and the doors were banging, the whole building was shaking.
"I saw some smoke later on, at the time I didn't realise what had happened. Then I was told the plane had crashed, it's terrible."
Tim Loughton MP, who represents East Worthing and Shoreham, tweeted: "Tragic news that there have been fatalities at the Shoreham Air Show crash and our thoughts and prayers must be with the families and casualties."
The A27 will remain closed until Monday, when it is expected to reopen from 06:00 BST, Highways England said.
There are currently delays of around 30 minutes between the junctions with the A24 South and the A2025.
It is the second incident at the Shoreham Airshow in recent years.
In September 2007 James Bond stuntman Brian Brown, 49, died when he crashed a World War Two Hurricane after carrying out an unplanned barrel roll at a re-enactment of the Battle of Britain.
The Hawker Hunter was a mainstay of the RAF through the 1950s and early 1960s.
First flown in 1951, the single-seat plane was used as a fighter, fighter-bomber for reconnaissance and for aerobatics.
There was also a two-seat trainer version, which served with many other air forces. Two-seater variants are still used by the RAF for training.
The Hawker Hunter was deployed in major operations such as Malaya and Suez.
The plane was also flown by an RAF display team dubbed the "Black Arrows".