Firefighters based at the fire station nearby had heard shouting in the street and went out to investigate, where they were confronted and threatened by the man.
The 26-year-old suspect remained hostile and struck the window of one of the first police cars when they arrived.
Eyewitness Jenilee Graham said she was unaware there was a situation developing until a woman stopped her and told her to get away.
The 23-year-old City worker, from south London, said:
"I was coming back from Asda with my mum when we saw a woman who told us to stop where we were going. We didn't know what was happening, but then we saw the man who the woman was talking about pop out from behind a bus. He ran towards us, then he darted down a side street. It was obviously pretty scary."
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Police were called to Kilburn High Road at 5.20pm after reports of a shooting.
In just 24 hours the offer was retweeted more than a thousand times on Twitter.
What a year it's been full of ups and down, turmoil and change and it seems events here in London have got the entire nation talking.