Just after 2 p.m., emergency services, including the London Air Ambulance, were dispatched to Maitland Park Road in Belsize Park in response to reports of a male stabbed in the area.
Scotland Yard said officers and paramedics discovered a 17-year-old with a stab wound.
The London Ambulance Service took him to hospital, where he is still in critical condition.
“A number of crime scenes are in place, and two weapons have been recovered,” a Met spokesperson said.
“In connection with the investigation, two people have been arrested.”
Anyone with information is asked to call police at 101 and reference CAD 3960/22Mar.
