Your camera is not the robbery object or else they would have snatched it from you.
You may be a "mark" of some initiation rite of passage of a teenage gang. Maybe the youngster was trying to "prove" his worth to his gang. That he dares to do this in daylight. Hence the unprovoked and senseless violence.
These kids will graduate on from tree branches to sharp edged weapons, gang fights at Boat Quay, alcohol, drugs, collecting protection money, etc...
The police will do the usual routine, ask you the same expected questions that don't result in the arrest of the gang. Don't expect them to go all out to hunt this group of xxxxxx teenagers down. Busting this kind of small time crime does not help their career.
But the police will take notice if you actually have the xxxxxx teenagers' photos and publish in the New Paper and it is seen by the big guns in the force. Then the authorities will be worried because there is an indirect racial side note that is potentially explosive and they will do something.