Police were called shortly before 8pm on Thursday, 1 July to reports of a man stabbed on Oxford Circus, close to Regent Street, W1.
Officers, the London Ambulance Service and London’s Air Ambulance attended. The victim was taken to a central London hospital where he remains in a critical condition.
A 25-year-old man was arrested after he was detained by members of the public at the scene. He has been taken to a south London hospital for treatment to a head injury, which is not believed to be life-threatening.
A crime scene is in place and officers remain in the area.
Anyone who witnessed the assault, or who captured it on mobile phone, is asked to contact police via 101 quoting reference Cad 7129/01Jul.
Alternatively, to remain anonymous, call Crimestoppers on 0800 555 111.