The Croydon underpass was closed earlier today after a teenager was seen to throw an item over the wall.
Plain clothed officers on patrol came across a group of youths close to Fairfield Halls at about 1.10pm.
One of the youths made off and threw an item into the underpass.
A police spokesman confirmed officers briefly closed the underpass on Wellesley Road and recovered a knife.
A male believed to be in his late teens was arrested on suspicion of possession of an offensive weapon and remains in police custody.
The road has since reopened.