Officers had been called to Heath Parade off Lanacre Avenue in Colindale, on Friday night to reports of a fight.
At the same time a 16-year-old attended Colindale police station with stab wounds and a 18-year-old was found at the scene with knife injuries.
Their injuries are described as serious but not life threatening. There have been no arrests.
A Metropolitan Police spokesman described the incident as gang-related.
Two teenagers have been stabbed in what police have described as a gang related street fight in north-west London.