Paramedics were called to the horrific scene in Victoria Road, Ruislip, just after 9pm on Tuesday.
A man believed to be in his 20s was rushed to hospital in a critical condition.
A Met Police spokeswoman confirmed the incident.
She said: “Officers, the London Ambulance Service and London’s Air Ambulance are in attendance.
“A male believed to be in his 20s has been taken to hospital in a critical condition.”
Dozens people have been stabbed in the capital since the start of the year.
Stabbings in London are at their highest level for more than half a decade, and have risen by more than 23 per cent on last year.
A total of 36 people have been stabbed to death in London since January 1, and 62 people killed by violent crime overall.
In March, the capital had a higher murder rate than New York City.
London Mayor, Sadiq Khan has criticised the Government’s approach to police funding and youth services, blaming them for the rise in violent crime.
He has said: “These statistics show the terrible repercussions around the country of a Government that has been desperately weak on crime for the last eight years, as well as weak on causes of crime.
“The most violent crimes, including knife crime, are rising across England and Wales.
“The Government must urgently increase funding for the police as well as restoring funding for there services that provide alternative paths away from crime, such as youth services, education, probation and mental health services.”
However, top Conservative politicians have blamed Khan for the spate of violence.
Tory Deputy Chair James Cleverly said on Twitter: “Sadiq Khan’s shocking complacency just isn’t good enough.”