A British Transport Police (BTP) officer was stabbed in the face as he responded to the terrorist incident at London Bridge.
Emergency services rushed to the scene following reports of a vehicle ploughing into pedestrians around London Bridge and stabbings at Borough Market shortly after 10pm.
The BTP officer, who was on duty, was one of the first on the scene after he responded to calls for help from the public, the force said.
A spokeswoman said it was believed he was targeted by one of the three attackers who were later shot dead by police.
BTP said in a statement: "He received injuries to his head, face and leg, which, at this time, are believed to be serious but not life threatening."
"The officer is currently being treated in a central London hospital and his family are with him while he receives treatment."
No other BTP officers are thought to have been injured in the incident.
Chief Constable Paul Crowther, who visited the officer with divisional commander Chief Superintendent Martin Fry, asked the public to stay "calm, vigilant and alert".
He said: "Again, our thoughts and sympathies are with those who have been injured in and affected by this horrific incident, including one of our own.
"We have implemented our strategic command structure and are working closely with the Metropolitan Police Service and other agencies in these early stages of the investigation.
"We would ask the public to remain calm, vigilant and alert, and to report any suspicious behaviour to the police on 0800 789 321."