A 20-year-old man was taken to hospital and is fighting for his life after being stabbed in Northolt.
Witnesses reported at least 12 police cars in the Wilsmere Drive area following the stabbing on Sunday evening.
Metropolitan Police officers were called to the residential road at around 7pm where they found the man suffering from stabbing injuries.
Both London Air Ambulance (LAS) and an air ambulance were called and the man was taken to hospital by road. A spokesman for the Metropolitan Police confirmed that the young man is in a critical condition.
Police arrested a man near the scene of the stabbing and he has been taken to a west London police station, where he remains.
Officers from Ealing CID are investigating the stabbing.
No further arrests have been made but police in the borough are continuing to make enquiries.