Officers rushed to the scene in Garter Way, Rotherhithe, just before 5pm on Saturday following reports of a man in his 50s behaving erratically.
When they arrived they saw the man was holding a knife.
The officers deployed a taser while trying to restrain the man but said the move was “unsuccessful.”
The man then suffered a self-inflicted knife injury to his throat and was taken to hospital by air ambulance, Southwark MPS said.
His condition at this time is not thought to be life-threatening.
Two officers were also taken to hospital as precaution after he spat at them.
Police are treating it as an isolated incident and said there is no concern for any wider threat to the community.
The man will be taken into police custody upon his release from hospital, the force confirmed.