He was standing just outside the police cordon when a white car sped by, police said. The driver did not stop and officers are now looking for them.
A spokesperson for the Metropolitan police said: “At approximately 02:35hrs an officer responding to the incident came in close contact with a car travelling past the scene.
“Fortunately, the car only brushed past the officer and did not fully collide with him. He wasn’t injured but was treated by paramedics for shock.
“Enquiries to locate the driver of the car involved in the incident with the officer are ongoing. No arrests have been made.
Efrayim Goldstein, a 30-year-old community reverend, came across the scene at around 2am on Thursday morning, as the stabbing victim was being loaded into an ambulance.
Mr Goldstein told the Mirror the officer signalled for the car to stop and it swerved.
He said he then helped the officer while a female officer and a London Ambulance Service worker tended to him.
“He was not breathing properly. He said ‘please help me,’” he said. “He was kneeling onto the other officer and then went back on to the ground.”
Police said the stabbing victim was taken to hospital but his condition was not life-threatening. The search for the knifeman is ongoing, they added.