Emergency services were called to the area of Station Road in Harrow around 8am on Saturday, following reports of a woman with a knife. Metropolitan Police have said two women sustained knife injuries and have been taken to hospital for treatment.
It is believed their injuries are minor.
A third woman was punched by the attacker, police have said.
Officers have said the attacker was detained and enquiries into the circumstances remain ongoing.
They also said that there is no indication that the incident is terrorism-related.
A Met Police spokesperson said: “Police were called at 08:09hrs on Saturday, January 22, to a report of a woman with a knife in the area of Station Road, Harrow.
“Officers attended and detained the woman. She has been taken to hospital for a Mental Health Assessment.
“It is believed that the woman inflicted slash wounds on two women and punched a third woman.
“Enquiries are ongoing to establish the circumstances.
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“The two women with knife injuries were taken to hospital for treatment.
“It is thought that both sustained minor injuries.”
A member of Harrow Online reported a heavy police and ambulance presence in the area following the attack.
They said they were covering the areas “by the Baptist Church and all up by Tyburn Lane – and at the front and back of the station”.