A sergeant from Islington arrived and was confronted by a 25-year-old man wielding a samurai sword, while a woman lay on the floor bleeding heavily.
After demanding he drop the weapon the officer was chased down the street by the man, who was attempting to strike him with the sword, Scotland Yard said.
But the officer managed to turn and step inside the sword’s arc before forcing the man to the ground and arresting him, police said.