The attack took place in Adelaide Close on the Moorlands Estate.
Scotland Yard said the victim was with his friend on the way to the friend's relative's house in Brixton when the attack happened.
The suspects were described as two young black men.
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The victim was hit as she crossed the road at a pedestrian crossing in Tulse Hill.
The 24-year-old was unarmed and the incident is not being treated as terror-related.