Two officers were taken to hospital with stab wounds to their hands after attempting to arrest a 17-year-old male.
The teenager was arrested on suspicion of attempted grievous bodily harm and possession of an offensive weapon.
One of the officers remains in hospital where he will have surgery on his finger, while the other has since been discharged.
On Tuesday (February 13) at around 2.25am, two officers on patrol in Acton High Street, west London, saw two males acting suspiciously.
When they pulled up alongside them, they ran off, prompting the officers to pursue on foot.
As one of the suspects – a 17-year-old boy – was being detained, he became aggressive and head-butted one of the officers in the face.
A struggle ensued as the officers continued to try and detain the suspect, resulting in both receiving stab injuries to their hands.
Other officers soon arrived and the suspect was arrested on suspicion of attempted grievous bodily harm and possession of an offensive weapon.
Both officers received first aid at the scene and were taken to a north London hospital.
One officer suffered facial injuries and a laceration to his hand, and the other suffered a stab injury to his finger, which requires surgery.
Ealing Borough Commander Chief Superintendent Paul Martin said: "This incident is a reminder of the dangers that police officers face on a daily basis while carrying out their duties protecting Londoners.
"Policing is by its very nature unpredictable – however no officer should be subject to such a violent attack.
"I have spoken with the officers who are in good spirits and we will support the officers and their families through this difficult time."
The suspect was taken to a north London police station where he remains in custody.