A French student was threatened with a knife and robbed as he made a call from a telephone kiosk.
The victim, 17, was speaking to a relative in France when the kiosk door opened and he was grabbed.
Offenders pointed a knife at him and stole personal belongings and a small amount of cash from his bag.
Members of the public called police and two men and a woman were arrested minutes later.
Detective Constable Martin Toff, of Brighton and Hove CID, said: "The victim was unhurt but extremely distressed."
Any witnesses to the mugging in Norfolk Square, Brighton, at 8pm yesterday are asked to call Mr Toft on 0845 6070999.
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