A man has been rushed to hospital by air ambulance after he was attacked in Harrow on Sunday afternoon.
An air ambulance was seen taking off from close to the location of the broad daylight attack that is now under investigation.
Armed Police officers from the Met Police attended to support local officers.
Police have established a crime scene within parts of Whitchurch Lane they have also cordoned off near to a parade of shops
A spokesman for the Met Police said
Police were called to Whitchurch Lane in Edgware at 6.37pm on Sunday, 19 July after a report of a man stabbed.
Officers and London Ambulance Service attended and found a man aged in his 20s suffering a stab injury.
He was taken to hospital where he is in a serious condition.
No arrests; Enquiries continue.
Road closures are in place while officers carry out on-scene investigations.
Anyone who witnessed the incident or with information is asked to call police on 101 via @MetCC and quote CAD7068/19 July.
To remain anonymous, please contact the independent charity Crimestoppers on 0800 555 111.