Elaine Costello, an aviation enthusiast from Romford but who is staying in Southwick, was recorded the Hawker Hunter plane as it performed a loop.
Seconds later, the aircraft slammed into the A27, just north of Shoreham Airport, erupting into a ball of flames.
Ms Costello, 50, said: “It was horrifying. I was totally shocked by it. I have never seen anything like it before.
“There was just a stunned silence from everyone in the crowd. We just couldn’t believe what we had all just seen.”
The crash took place at about 1.20pm, with emergency services reporting that the plane hit a number of cars on the A27, leaving several people seriously injured.
Describing the scene, Ms Costello said: “There were just flames and black smoke coming out.
“It was horrible to watch.”
Ms Costello said there was a great deal of confusion, from spectators, in the aftermath of the tragedy.
Airshow organisers have told people to remain at the airport.
Traffic is also reported to have been seriously affected, with the A27 closed in both directions.
The cause of the crash is unknown. However, air crash investigators have been informed and will conduct an examination into the incident.
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