Mohammed Hussein, 29, dashed out of a pizzeria where he works part-time when he heard a scooter crash at speed.
Minutes earlier a six-strong gang had stolen watches worth hundreds of thousands of pounds after using axes to smash display cabinets at the nearby department store.
Mr Hussein found two men lying in the street. The gym fanatic said: “One of the men was wearing a burkha and was lying under the scooter – he was motionless.
“The other man stood up and was pretending the one on the ground was his wife – he was saying, ‘Help my wife, help my wife.’
“But the person lying on the floor was clearly a man – he was huge and you could see his hairy chest peeping through the burkha. I noticed a bag on the floor and lots of expensive-looking watches – gold, silver, bronze.” When the suspect tried to flee, Mr Hussein fended off his blows and managed to restrain him with the help of a security guard from a nearby restaurant.