A police dog handler had his leg broken in an altercation after challenging three men about their behaviour.
The Hertfordshire Police officer was patrolling in a police car on Sunday in St Peter's Street, St Albans, when he saw the men shouting abuse at a couple.
During the following fracas at about 1800 GMT, the officer fell and broke his leg and was taken to hospital.
Later three men aged 20, 30 and 31, who live locally, were arrested and are in custody at St Albans Police Station.
Pc Paul Gillick, of Hertfordshire Police, said: "This is an unusual incident in which an officer has sustained serious injuries which have required surgery.
"We need to hear from people who were in the town centre on Sunday evening and who either saw the group prior to the incident taking place or the fracas itself."
Anyone with information is asked to contact Hertfordshire Police.