Officers rushed to De Havilland Road in Burnt Oak at 5pm on Saturday following reports that a group of youths were tampering with a moped.
Upon seeing the police, the group ran away from the officers.
A chase resulted in a 15-year-old boy being detained. The boy was arrested and taken into custody before being bailed.
Enquiries remain ongoing as the police continue to search for the other youths.
During a search of the area, officers found a bag believed to have been discarded by one of the group.
This bag contained a loaded firearm, a large Rambo knife and a quantity of Class A and Class B drugs. The firearm and drugs will be analysed in the coming days.
Detective John MacLeod, of the Met’s Specialist Crime unit, said: “This case shows how officers will respond to information received from members of the public.
“The age of the youth arrested, and in all likelihood the other suspects, is significant.
“Officers often hear from those critical of the police that stopping and searching young people whose behaviour has caused suspicion is not acceptable.
“However cases such as this clearly demonstrate why officers must continue to protect our communities by investigating and challenging suspicious behaviour, even if the suspected offenders are very young.
“We need to build young people’s confidence in the police.
“And I hope that taking a firearm, a large knife and illegal drugs out of the hands of children is exactly what the vast majority of young people and all communities in London expect of the Met as we work to keep people safe and continue reducing violent crime.”
