An off-duty policeman fought off the London Bridge terrorists with his bare hands, it emerged, as Scotland Yard revealed two more of its officers were injured in the attack.
A British Transport Police (BTP) officer was also stabbed in the head, face and leg after taking on the three knifemen armed only with his baton.
Metropolitan Police Commissioner Cressida Dick said she was "enormously proud" of the unarmed officer who stepped in to help uniformed colleagues and members of the public.
"He had finished a day's work and was having a drink with friends and he witnessed the attack," she told BBC Breakfast.
"My understanding is that, without hesitation, wearing his normal clothes, he dived in and tried to assist, and he I'm afraid was himself severely injured, but utterly heroic."
The officer, who was based in Southwark, remains in hospital in a serious condition, while two other Met officers, who were both on duty, received less serious injuries during the Borough Market rampage.
Police said a plain-clothes officer received stitches to a head injury, while a uniformed officer suffered an injury to his arm.
The BTP officer, who was on duty, was one of the first at the scene after he responded to calls for help from the public and suffered serious injuries, which the force said are not thought to be life-threatening.
Ms Dick described the police response as "extraordinary", adding: "I think those of us who weren't there are in awe of what both the police officers armed, the unarmed officers we've heard about and indeed the emergency services and members of the public - just extraordinary amounts of courage shown."