Emergency services descended on a leafy north London high street following reports of a “bad smell”.
Scores of shoppers were evacuated from businesses in West End Lane, West Hampstead, after residents reported a “noxious” stench.
Dramatic images posted on social media showed a huge police presence on the road just after 9am on Monday.
London Fire crews were also called to deal with the situation.
Witnesses said they had been told to “move back” by police officers amid concerns the odour was being caused by ammonia.
A spokesman for Scotland Yard told the Standard: “We were called to reports of a noxious smell coming from one of the buildings in West End Lane.
“A cordon is in place while we investigate and make sure the area is safe.
“The incident is not thought to be terrorism related.”
The area was deemed to be safe just after 12.30pm.
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