The Singapore Grand Prix was interrupted with around 20 laps to go when a member of the crowd stepped out on to the track.
Photographs and TV footage showed a man in a dark top walking along the inside edge of the track as cars flew by just a few feet away.
Race leader Sebastian Vettel was among the first to sound the alarm as he drove past the spectator, shouting into his team radio: "There is a man on the track! A man on the track!"
Organisers quickly deployed the safety car, as the man was seen reaching a gap in the fence and jumping away off the circuit.
Many racers took the safety car opportunity to pit, including the front three of Vettel, Daniel Ricciardo and Kimi Raikkonen.
The incident, which occurred on lap 37, threw teams' tactics into chaos and, with the cars all bunched up waiting for the safety car to get out of the way, Jenson Button clipped the back of Pastor Maldonado's vehicle, losing his front wing.
With 21 laps to go the safety car moved out of the way, allowing racing to resume with Vettel in the lead.
