The victim was seen falling to the ground in Market Place near the junction with the famous street shortly after 5.30pm. Photos from the scene show a cordon in place as emergency crews treat the victim. Two ambulance crews and an air ambulance were sent to treat the man and he was rushed to a major trauma centre.
A Met spokeswoman said: “Officers attended and found a male suffering from a stab injury.
“Emergency services remain at the scene.
“There have been no arrests.”
The Met later issued a second statement saying: “Police were called at 5.38pm today to reports of a male stabbed on Market Place close to Oxford Street, W1.
“He has been taken to hospital and we await an update on his injuries.
Photos taken at the scene show shocked shoppers gathered near a police cordon.
One snap appears to show officers treating the victim.
One man who posted a video of the scene on Twitter claimed to have seen someone “running down the middle of Oxford Street in broad daylight on a Saturday, which is today, with a machete, chasing someone.”