Debris was scattered across the main straight after a collision between Alfa Romeo's Antonio Giovinazzi, McLaren's Carlos Sainz and Haas's Kevin Magnussen as the race re-started following a first safety car period.
Valtteri Bottas was leading at the front for Mercedes, with team mate Lewis Hamilton second, and controlling the pace.
As the Finn prepared to accelerate away, the unsighted back-markers appeared to react as if the leaders had already re-started the race. Sainz hit the back of Giovinazzi's car as the Alfa swerved.
"I think people were going before you'd gone," Mercedes told Bottas.
Sainz reported that he and the others were OK. "The crash was really scary," he said.
The safety car was immediately deployed again and the race then halted.
"That was...stupid from whoever was at the front," exclaimed Haas's Romain Grosjean. "Do they want to kill us or what? This is the worst thing I've seen, ever."
The safety car had been deployed on the opening lap when Pierre Gasly, shock winner of last Sunday's Italian Grand Prix at Monza for AlphaTauri, was launched into the air and took out Red Bull's Max Verstappen.
It was the second race in a row to be red-flagged. (Reporting by Alan Baldwin, editing by Ed Osmond)