Police have arrested a man on suspicion of assaulting a suspected shoplifter, after footage of the incident went viral.
Scotland Yard said it was "made aware" of footage of an assault being viewed online that was believed to have taken place within a branch of Costcutter in Newham, east London.
In a statement, Met Police said: "Following initial inquiries police have arrested a man in his early 20s on suspicion of common assault.
"He was taken to an east London police station where he remains at this time."
Anyone with information is asked to contact police.
In the footage, a man is seen slapping another and appears to say: "If you steal from this shop again you'll get slapped up again. Learn from it bruv."
In a statement, Costcutter said one of the store owner's relatives - who was not an employee - "took matters into his own hands when the owner was absent".
"We fully appreciate the level of concern people are expressing which we share.
"We want to make it absolutely clear that we find the events within this video totally unacceptable and there is absolutely no place for it within our business.
"Having spoken to the police, it is our expectation that they are going to investigate this thoroughly and we will take whatever action is necessary as a result."