Detectives are questioning a man in connection with a knife attack on two police officers who were stabbed by a bus passenger as they made routine ticket checks.
A police constable and a community support officer (PCSO) were attacked after they discovered a commuter 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 Wednesday.
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, underwent surgery and was in a "serious but stable condition" after his throat was slashed.
Colleagues of the injured officers, who are based in Ealing Borough, arrested the attacker at the scene. He was taken to a west London police station where he was being held in custody.
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. Another eyewitness said the suspected knifeman had been travelling on a 207 bendy bus which travels between Hayes and White City.
The attack took place close to Christ Church School and a short distance from Ealing Town Hall in a street packed with shoppers.