One person was arrested following the incident on Seven Sisters Road in Finsbury Park.
Officers were called at 12.20 BST and remain at the scene, the Metropolitan Police said, describing it as a "major incident".
London Ambulance Service said it had sent "a number of resources".
Witnesses have described seeing police tend to the injured.
Cynthia Vanzella said on Twitter: "Horrible to watch police officers doing cardiac massage at people on the floor, desperately trying to save them. I just hope they did."