An inmate has been stabbed at HMP Wandsworth, in the same week that a terror suspect escaped from the prison.
The man was taken to a major trauma centre by ambulance in a critical condition after emergency services were called around 3pm.
An air ambulance was also sent to the scene.
Police later on Sunday said the man's condition was not life-threatening.
It is understood that the incident involved two prisoners.
Police have said they were called at around 3.20pm to reports of an inmate being assaulted at the prison.
They added that no arrests have been made and enquiries are ongoing.
A Prison Service spokesperson said: "Staff swiftly responded to an incident between prisoners at HMP Wandsworth today.
"We have a zero tolerance approach to violence and will always take strong action against those who break these rules."
A father who was visiting his son inside the prison said the stabbing happened in one of the communal areas.
He told Sky's Tom Parmenter: "I saw two ambulances going in and someone said a prisoner has stabbed someone.
"It's just another day at Wandsworth jail - you do worry as a parent.
"It is chaotic right now - even the officers inside all say they need more staff, my son is always calling saying he is on a lockdown meaning they can't leave their cells at all, not for a shower not for anything.
"It's unacceptable, yes they have to be punished but it is inhumane."
The stabbing comes days after Daniel Khalife, a 21-year-old terror suspect, went missing from the prison on Wednesday.