Gatwick Airport's North Terminal was evacuated "for the protection of the public" on Saturday after a suspicious item was found, the media reported.
Hundreds of people were photographed standing outside the airport in the rain after being told to leave the airport at around 10 a.m.
Witnesses said armed police arrested a man believed to have a gun in his bag.
A spokesperson for Sussex Police told The Independent that she could not confirm the reports of a security alert but said: "The terminal is being evacuated at the moment. It is for the protection at the public."
She said the evacuation was a precautionary measure sparked by the discovery of a "suspicious article".
The evacuation comes amid heightened security in Britain following a wave of terror attacks in Paris on Friday night that killed 127 people.
