The horrific incident, which took place shortly before 6pm on on Alderman’s Walks, is the latest violent attack to take place in the capital this year.
Liverpool Street has been closed to traffic and pedestrians following the attack, although the train station is still open.
The victim’s identity has not been revealed by police, but he is understood to be in a serious condition at a nearby hospital.
Liverpool Street is one of the busiest areas in the City of London, the capital’s financial hub, and was busy with commuters at the time of the stabbing.
A spokeswoman for City of London Police said: “Police were called to a man being stabbed near Liverpool Street station on Alderman’s Walks at 5.59pm.
“The man has been taken to Royal London Hospital in a serious but not critical condition.
“Liverpool Street has been closed but the station remains open and accessible to the public.”
No arrests have been made in connection with the stabbing.
The attack comes as the government and London’s mayor Sadiq Khan struggle to control the soaring levels of violence in the capital.
Cressida Dick, Britain’s most senior police officer, recently spoke of her anger at the sheer volume of lives lost due to knife crime.
The Metropolitan Police Crime Commissioner said in February: “I have spoken all too often at the anger I feel at the senseless waste of life.
“The frequency with which some of our young people are prepared to take each other’s lives is shocking.”
Tory MP and ex-police minister Sir Mike Penning told the Daily Express that a major crackdown on knife crime was needed to tackle the issue.
He said: “Police chiefs have got to give their frontline officers the confidence to stop and search, while the CPS have to throw the book at those who carry and use knives.
“That’s the only way we will have a deterrent. Unless that happens, we are going to be in for a very difficult summer and years ahead.
“Nearly all the problems surrounding knife crime are down to drugs and gangs. Police chiefs have to understand the public is not going to put up with this any longer.”