Officers formed a human barrier outside Colindale police station after protesters gathered over the arrest of a 14-year-old boy.
In a statement, Metropolitan Police said officers arrested the teenager on suspicion of possessing cannabis following a stop and search.
During the arrest, a group gathered around the officers and a further two men, aged 23 and 25, were arrested on suspicion of obstructing police.
The officers then left the scene in a car and were followed on foot by 40 people to Colindale police station, the force said.
The protesters are now protesting at the police station and are demanding the release of the boy.
This is @TemiMwale Director of @4FrontProject outside Colindale police station right now. Police have snatched a 14yo black child off the street and won't say why.
Watch the video, share it and if you have any video footage from today please get in touch with @4FrontProject 1/5 pic.twitter.com/rhodZYLSQu— Barnet Transformed (@barnettransfrmd)August 7, 2020
A cordon is in place around the station and police have authorised a Section 35 dispersal order.
In a video posted to Twitter, Temi Mwale, director of youth group 4FrontProject, claimed that two members of her staff had been arrested by police.
— Simon Vessey (@Simon_Vessey)August 7, 2020
Ms Mwale appealed to members of the public to join the protest outside the station.
"This community is sick and tired of the way we are being treated, now we need your support," she said.
"They have disappeared members of our community, they have disappeared young people, they have disappeared community workers."
4FrontProject describes itself as a member-led youth group empowering young people and communities to fight for justice.