A 27-year-old man has been arrested by local police after walking onto the circuit along the Esplanade Drive during the Singapore Grand Prix.
The man climbed onto the track from the outside of the circuit and walked across, shortly before race leader Sebastian Vettel passed through.
The man then climbed over the barrier on the other side of the circuit, while the Safety Car was deployed for the second time in the race.
"I had to look again because I wasn't sure whether I had a problem with my eyesight or I saw somebody crossing the track," said Vettel.
"I went on the radio and said “some guy's running across the track!” I think I saw him taking a picture. I don't know. I hope it was a good one at least. I hope it was in focus!
"I wouldn't cross the track if I was him. It was crazy, obviously, and we had a Safety Car straight away. Maybe in the next year we need to block the grandstands a bit better. Fortunately nothing happened to us and nothing happened to him – as far as I know."
It is the second occasion when a person has breached security in 2015, after a spectator crossed the circuit during practice for the Chinese Grand Prix in an attempt to enter the Ferrari garage.