The man's body reportedly laid on the street outside Lloyds Bank for several hours.
"A group of guys were sitting in a car by the bank and a big guy came up to them with a shotgun," a shopkeeper from the area said.
"They all got out of the car and ran so the guy decided to damage the car with the back of the gun.
"As he was hitting the windows the gun went off and he shot himself - that's what we've been told."
"A man in his 20s was found with gunshot wounds and he died at the scene at 3.50pm," a statement from Met Police said.
"Police remain at the scene – there have been no arrests at this stage and enquiries in to the circumstances continue."