Three Jewish boys were assaulted and threatened with a knife at Hampstead Underground Station on Monday evening, Shomrim NW London reported on Tuesday.
One of the victims was admitted to the hospital for observation.
The suspects are believed to be a group of 6-7 men, Shomrim added.
Community Security Trust said it was aware of the "appalling racist assault" and is in contact with one of the victims' families to provide support.
Growing threat facing Jewish communities
"This report is yet another stark reminder of the growing threat facing Jewish communities, including children," said Campaign Against Antisemitism.
Hampstead has an established Jewish community: in the 2021 UK Census, 11% of Hampstead Town identified as Jewish, and 43% identified as Jewish in Hampstead Garden Suburb.