Schoolgirls in Brent and Harrow are being urged to "stay safe and be vigilant" as a man is approaching pupils who are on their own.
Police are linking two incidents in the last week where an unidentified male has approached schoolgirls walking home on their own..
Officers said in a Nextdoor online group community post the description of the man "is vague at this time" but is white and about 25 years old.
A spokesperson said: "The two incidents are being considered as linked."
Police advise girls to travel in groups where possible and stick to busy roads.
Pupils should ensure they keep charged mobile phones with them and inform family and friends when leaving school so that they will have an idea of when to expect them when they get home.
Students are urged not engage with strangers who approach them.
"They should run away and call 999 immediately if they fear for their safety," police added.
"Police should be contacted as soon as possible so that the circumstances can be investigated at the earliest opportunity."
"If any direct contact is made with clothing then keep this as there may be forensic opportunities for the police to progress."
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