A police officer was stabbed in a "frenzied" machete attack as he tried to stop a van in east London.
The PC - described by colleagues as "amazing" - managed to Taser his attacker while being stabbed in the head and body.
He is in hospital with multiple injuries but the Met says he will recover.
A 56-year-old man was arrested at the scene in Leyton on suspicion of causing grievous bodily harm.
Det Ch Supt Richard Tucker described the attack, which happened shortly after midnight, as "frenzied, unprovoked and shocking".
The two uniformed officers suspected the van was being driven without insurance when they tried to stop it at the junction between Coopers Lane and Leyton High Road.
When they spoke to the driver he became "quite aggressive" and returned to his van before "a violent struggle", Det Ch Supt Tucker said.
The injured PC is a patrol officer who has been with the force for about 10 years.
Det Ch Supt Tucker said: "He told me he was OK but that's the type of character he is."
Prime minister Boris Johnson said the attack "underscores for me the bravery of our police, people who actually go towards danger to keep us safer."
Muhammad Faisal, who lives nearby, said he was woken up by a noise outside his flat and saw someone lying in the road.
Mr Faisal said he initially feared the officer "had lost his life", but he began to move and speak to colleagues after a few minutes.
Minicab driver Mr Faisal reported seeing "a lot of blood" on the ground as fellow officers tried to stop their colleague bleeding.
He said he also saw a suspect being held face down on the floor before being taken away in a police van.
Insp Julia James said: "This was a sudden and brutal attack on a uniformed officer carrying out their duties.
"What began as a routine vehicle stop has transformed very quickly and unexpectedly into an unprovoked attack with a weapon."
The van stopped by the officers has been taken away from the street for forensic examination.
Home Secretary Priti Patel tweeted that she was "absolutely horrified by the shocking stabbing", while London mayor Sadiq Khan said he was "utterly appalled by this sickening attack".
"This was an attack on a police officer, who showed tremendous bravery by continuing to defend others even whilst being attacked himself," he said.
Ken Marsh, chairman of the Met Police Federation, said it was "horrific news".
"Police officers should be going home at the end of their shifts, not to hospital," he said.
The Met has urged witnesses to get in touch.