A police officer is in a critical condition after being stabbed a number of times with a machete in a "sudden and brutal attack", Scotland Yard has said.
The patrol officer was stabbed in the head and body in Leyton, east London, at around midnight on Thursday, after he attempted to stop a van at the junction of Coopers Lane and Leyton High Road.
The vehicle's driver, who was in his 50s, failed to stop and then got out of the van and injured the officer with the machete, police said.
The injured officer, aged in his 30s, managed to Taser his attacker, before being taken to hospital by ambulance.
He is in a critical but stable condition, but "will make a recovery", according to his bosses.
The driver, who did not require hospital treatment, has been arrested on suspicion of grievous bodily harm and was taken to an east London police station.
Police have said there is nothing to suggest the incident is terrorism related.
North East Command Unit's inspector Julia James, said: "What began as a routine vehicle stop has transformed very quickly and unexpectedly into an unprovoked attack with a weapon.
"I would like to commend his bravery and resilience especially as he managed to draw a Taser, despite being repeatedly stabbed, to prevent the suspect harming anyone else and to protect himself from further injury."
WATCH: Alison Cope, whose son was stabbed, shares her thoughts on the incident
Responding to the incident, London Mayor Sadiq Khan tweeted: "This sickening attack on a serving police officer is utterly appalling.
"The men and women who serve in our police force in London are the best in the world, and their courage and dedication is inspiring. I'm in contact with senior Met officers, and an arrest has been made."
And Home Secretary Priti Patel, said she was "absolutely horrified" by the attack.
Police have asked anyone who witnessed the incident to call 101 or Crimestoppers anonymously on 0800 555 111 and quote reference CAD3/8August.