A teenager confessed to carrying a knife with a 12cm blade in Harrow town centre for "protection" as he has "beef" with people in Northolt, a court heard.
An 18-year-old was found guilty of possessing a huge knife in the busy shopping area - after running from police officers patrolling gang-ridden areas.
Willesden Youth Court heard on Monday (December 4) how the teenager, who was 17 at the time and cannot be named for legal reasons, was spotted walking with two friends when he saw officers who recognised him.
Giving evidence at trial, PC Domeney told the court: "I was on duty on October 1 as part of a proactive team out on patrol in the town centre.
"We patrol any areas associated with gang activity."
Knife crime epidemic
The officer recognised the defendant and one of the other boys, before getting out of his vehicle and calling his name, asking him to "stay there".
"At which point he ran off," the PC added.
He said: "He ran from College Road, across Station Road and up an alleyway, which is behind Lyon Road. The alleyway leads to the service road behind shops.
"I chased him and as we entered the service road I saw his left arm making a throwing motion behind William Hill betting shop.
"I heard a 'clang' sound, which sounded like metal hitting concrete.
"I continued to try and apprehend him but unfortunately he's faster than I am."
The teenager eventually outran the officer, who asked the public for help, before heading back to the betting shop.
The huge kitchen knife had a 12cm blade and was produced in court as evidence. It had been spotted by a member of William Hill staff who pointed it out to PC Demoney.
Upon seeing the knife, District Judge Brennan said: "There is certainly a substantial blade there."
The court also heard from the teenager's Youth Offending Service case manager, who alleged that he confessed to her that he had the knife on the day in question, during one of their regular meetings.
She told the court: "He said the reason I had the knife on me was because I was seeing a friend in Northolt and I have beef up there and needed it for protection."
The teenager was arrested the day after the police chase, on October 2, at the rear entrance at Harrow-on-the-Hill station.
The teen had given a no comment interview after receiving legal advice, he said.
When asked by prosecutor, Beata Murphy, why he ran from police, the defendant said: "I just panicked... I didn't know what to do at the time."
'You did have the knife and I find you guilty'
Addressing the defendant, District Judge Brennan said: "I've heard from you that on October 1 you panicked when you saw officers and ran off.
"Having listened to evidence from PC Domeney he doesn't say he saw the knife leave your hand.
"What's telling for me is he hears a clanging sound between two cars."
He added: "He was five metres behind - it was a short distance between you and the officer. I think he's telling the truth.
"She struck me as someone who came along to court to tell the truth and it leads me to the conclusion you did have the knife and I find you guilty."
The 18-year-old had previously pleaded not guilty to possessing a knife blade/ sharp pointed article in a public place, at a hearing on October 10.
He will be sentenced at Willesden Youth Court on January 11.
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