Seven people have died after a plane crashed in to cars on a major road during an aerial display at the Shoreham Airshow in West Sussex.
The aircraft, a historic Hawker Hunter jet, plummeted to the ground when it failed to pull out of a loop manoeuvre, smashing into the A27 in a huge fireball.
The South East Coast Ambulance Service said: "SECAMB confirm 7 fatalities from the crash at the Shoreham air show today. Thoughts are with the families and loved ones."
West Sussex Police said a casualty is being treated at the Royal Sussex County Hospital with serious life-threatening injuries, while a further 14 people have been treated for minor injuries, one of whom this evening was released from hospital.
It is not known whether the pilot was able to eject.
All the casualties are believed to have occurred on the A27, and there are not thought to be any injuries to anyone at the airfield.
A police spokesman said: "A number of people are known to have died when a historic military jet crashed into a busy main road at Shoreham in Sussex.
"The Hawker Hunter was taking part in an air show at the airport adjacent to the A27 when it crashed on to the road and ended up in a bush at 1.20pm.
"Emergency services have declared a major incident and a number of helicopters have been deployed to the scene to evacuate casualties to hospital.
"South East Coast Ambulance Service have confirmed that there have been seven fatalities declared at the scene, one patient with serious life-threatening injuries has been transported to Royal Sussex County Hospital and a further 14 patients treated for minor injuries.
"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.
"West Sussex Fire and Rescue Service has sent 10 appliances to the scene.
"The Air Accidents Investigation Branch 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."
The A27 has been closed at Lancing and to the north of Hove, police added, and is expected to remain closed for some time.
Visitors to the show were kept at the airfield until about 7pm before being directed away from the site.
Tomorrow's air show has been cancelled, organisers said.
Prime Minister David Cameron sent his "heartfelt condolences" to the families of those who died in the crash.
A Downing Street spokesman said: "The Prime Minister sends his heartfelt condolences to the families of the people who were so tragically killed after a jet crashed into several vehicles in West Sussex.
"The Prime Minister's thoughts and prayers are with the families and friends of the victims."
Horrified onlookers described seeing the plane flying low to the ground followed by a "huge ball of fire and black smoke".
Footage of the crash posted online showed the jet carrying out a loop manoeuvre high in the air but failing to pull up in time.
There appeared to be queueing traffic on the A27, and then the plane slammed into the ground and burst into a ball of flame and smoke. The pilot did not appear to deploy the ejection system.
Photographs taken by witnesses showed the burning aircraft ploughing across the road just yards from cars and a group of people, its canopy open and wing ablaze, engulfing traffic lights and roadwork cones and signs in a huge ball of fire.
Burning debris was also visible high in 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."
Laura Raymond, a presenter for Splash FM, said the accident happened less than a quarter of a mile from where crowds and families were watching the event.
She said a "couple of planes" were taking part in a jet display when one appeared to fly too close to the ground before onlookers saw a "huge ball of fire and black smoke".
She told Sky News: "We were watching the displays ... and the fast jets were going on. To the best of our knowledge the Hawker Hunter T-7 display was under way.
"Within seconds we were thinking, 'gosh, that plane is going rather low'. There was a ball of fire, we didn't even hear an impact, and then plumes of black smoke."
Ms Raymond said commentators at the event described the situation to the crowds as a "tragic incident", and that the incident was outside the airfield boundary.
An Air Accidents Investigation Branch (AAIB) spokesman said: "The AAIB is aware of the incident and we will be deploying a team to investigate."
Tim Loughton, MP for East Worthing and Shoreham, said his thoughts were with those affected.
He said: "It's tragic news that it has been confirmed that there have been fatalities and we obviously hope that those numbers will not increase.
"Given the many thousands of people attending the air show we thank God that the aircraft did not hit a larger crowd. Our thoughts and prayers are with those affected."
Mr Loughton drove past the entrance to the air show a few minutes before the crash.
He said: "There was about a couple of hundred yards of cars queueing at the entrance to get in as well as stewards and some spectators sitting by the roadside. It's horrific. It is a horrendous crash."
It is the second tragic incident at the Shoreham Airshow in recent years.
In September 2007 James Bond stuntman Brian Brown, 49, died when he crashed a Second World War Hurricane after carrying out an unplanned barrel roll at a re-enactment of the Battle of Britain.