Singer Ellie Goulding came to the aid of a driver whose car was being pushed sideways along a road by a lorry.
Footage shows a Volkswagen GTi being pushed down Western Avenue, A40, by a Royal Mail delivery lorry near the Greenford roundabout in west London.
Goulding posted on Instagram to criticise other drivers who got out to film the crash and "shout abuse" at the lorry driver.
The Royal Mail says it is investigating the crash.
The truck driver appears astonished to see the car in front of his vehicle, claiming he did not see it, or know it was there.
He can be heard yelling: "I didn't see him, I honestly didn't see him."
Goulding told her 14.4 million Instagram followers: "On a side note, I can't believe the first instinct of the other drivers who got out was to instantly start filming on their phones and shout abuse at the poor shocked driver, not even checking the other driver was okay.
"What on earth."
Goulding told BBC Radio 1 she intervened because "no-one was stopping".
She said: "I think people were desperate to get to work. All these people were just driving on.
"We just drove up right next to it [the lorry] to be like 'Mate, you've got a car on you!'"
The driver who was shunted along the road later messaged the singer "to just say he was OK," she added.
The Met Police said there were no reported injuries and no arrests have been made.
A Royal Mail spokesman added: "We are very concerned about this incident. We sincerely hope that no one was hurt. We are investigating as a matter of urgency."
Road safety campaigner Rebecca Ashton told the Victoria Derbyshire programme she hoped it was not a stunt.
She said: "He must have been able to hear the scraping of the tyres - possibly a feeling of pushing a car."