Two police officers were stabbed by a man after they stopped him to check he had paid his bus fare.
The police constable and police community support officer (PCSO) were attacked after they found out the man in his 30s was wanted for a recall to prison.
The attack happened at a bus stop in New Broadway, a busy shopping street in the centre of Ealing, west London, at about 3pm on Wednes.
The 27-year-old PCSO was treated in hospital and later released after suffering stab wounds to his head as he wrestled with the attacker.
The injured police officer, 23, had his throat slashed and he is in a “serious but stable condition” and is undergoing surgery.
The alleged attacker was arrested at the scene and taken to a west London police station.
Chief Superintendent Andy Rowell, the Ealing Borough police commander, said the incident occurred when a man began “behaving suspiciously” after being approached by the PCSO.
On being challenged he allegedly turned his knife on the PCSO and the officer who suffered “multiple stab wounds initially thought to be life threatening”.
Eyewitnesses said the officers, who were wearing stab-proof vests, were part of an operation checking bus tickets.
One officer said the suspect was stopped and when a radio check was made the pair learned he was wanted for recall to prison. The source added that when it became clear he was about to be arrested the man pulled out a knife and attacked the officers.