Wembley police are linking two incidents where two 12-year-old girls were followed by a man who "grabbed" one of them by the arm.
On March 11 a 12-year-old girl was followed by a man in Dobbin Close, Harrow, at around 2.10pm.
She managed to lose him near to Belmont Circle, police said.
Five days later on March 16 at a second 12-year-old girl was approached by a man on Neasden Lane at 4.20pm.
He grabbed her arm before running off.
In both cases, the suspect is described as white, slim , about 25 years of age, and was wearing a black zip-up hoodie, and blue facemask.
Schools in the vicinity have been advised of the two incidents and general safety advice has been provided.
DI Jonny Newell, from the North West Command, said: “I know that parents and local residents will be rightly concerned about these incidents, but I would like to reassure them that we are carrying out a number of enquiries in an attempt to trace this individual and continue to provide enhanced patrols within the area.
"I would encourage anyone with specific concerns or who wishes to report a similar incident to approach a police officer or make contact via 101.
"In an emergency always call 999.”
Anyone with information should call 101, quoting CAD 5171/16MAR21.
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