The incident happened in High Road just after 5pm on Thursday (October 9).
Officers arrived with paramedics who treated the teenager before taking him to hospital.
It has been confirmed that he is still in a critical condition.
There have been no arrests but an investigation into the incident has been launched.
Huseyin Kaylan has owned New Southgate Food Centre in High Road since 2019.
Huseyin Kaylan (Image: Huseyin Kaylan) He said: “I was in the shop and I heard a gunshot so I approached a window where I saw him walking slowly towards the shop bleeding.
“I dragged him inside to safety and tried to put pressure onto his wound whilst calling the police and ambulance.
“I wasn’t hesitant at all – it’s in my nature that whatever happens, I will help.”
Huseiyn was in the shop with one other colleague when the shooting happened.
He said this wasn’t the first time he has been aware of violent activity and a growing gang presence in the area.
“This area is getting chaotic – it’s not the first time my shop has been closed due to crime,” he said.
“It feels in general, London is going downhill and young generations are seeing killings everywhere and it’s affecting them.”
CCTV image of New Southgate Food Centre after the teen was taken to hospital (Image: Huseyin Kaylan) New Southgate Food Centre was closed until yesterday evening (October 10) as it had to be investigated as part of the crime scene.
The shop has since been nominated for the National Lottery Retail Champions Award by local Alex Mankowitz for the “exceptionally kind behaviour” as documented in the nomination form.
Met detectives are appealing for information to help with their investigation.
Detective Sergeant Catherine Dempster said: “We recognise the shock and concern this will have caused the local community in Arnos Grove and are working at pace to identify the perpetrators.
“This cannot be done without assistance from the public.
“Were you in or near to High Road, Arnos Grove around 17:00hrs? Do you know anything about this incident?
“Did you hear gunshots? Do you have any CCTV / dashcam footage within the vicinity of the incident? If so, please come forward.”
If you have any information at all that may assist police, please call 101 quoting 5749/09OCT.
Alternatively, you can contact Crimestoppers anonymously on 0800 555 111.

