A 75-year-old man was sprayed with a noxious substance by two men wearing motorcycle helmets in a "despicable attack".
The elderly man was approached by two men on a scooter who set upon him before grabbing his wrist and stealing his luxury watch.
One of the men then "casually walked up to him and squirted a noxious substance at his face from a bottle", according to Metropolitan Police .
The incident occurred at Buckingham Parade, in The Broadway, Stanmore , at around 5.32pm on Wednesday (September 20).
The elderly man was treated at the scene by paramedics from London Ambulance Service and a doctor who was passing by.
The thieves made away on an all-black moped with the man's customised Cartier Pasha wristwatch, which has a brown leather strap, gold rim and gold face and was engraved with hour numbers.
Both suspects wore black clothing made of a "canvas/tactical material" and were wearing rounded helmets with visors completely covering their faces.
No arrests have been made and police in Harrow are appealing for anyone who may have information about the incident to get in touch.
DS Amar Patel from Harrow CID, who is leading the investigation, said: "This is a despicable attack on an elderly gentleman going about his daily business.
"The victim was unable to defend himself against this vicious robbery which was following up by a senseless attack with what we now know to be a noxious substance.
"A specialist has today - Thursday September 21 - confirmed that were it not for the swift intervention and treatment he received from people who attended to him at the scene, he may well have lost his sight in one eye.
"It is vital that we bring the culprits to justice before they target anyone else. They may well have used local petrol stations."
Anyone with information on this attack is asked to call police on 07769 162 964 or Crimestoppers anonymously on 0800 555 111.
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