A Metropolitan Police spokeswoman said Threadneedle Street could be closed for “several hours”.
She added: “At 1.55am on Threadneedle Street a 26-year-old man was stabbed the man was taken to hospital by London Ambulance Service and is currently in a stable condition.
“Police are investigating the incident.
“Any witnesses are urged to come forward on 0207 601 222.”
Police did not say how many people were involved in the violent incident.
Five officers from City of London Police are at the scene and commuters travelling through Threadneedle Street should expect delays as buses using that route are being diverted.