The incident occurred in Crawley town centre at At 1.30am today (Saturday, April 7).
Two officers in a police car were driving along Peglar Way when they saw two men apparently fighting in the rear car park of The George Hotel.
They intervened and one of the officers, a local uniform Sergeant, sustained a serious knee injury, police said.
He is currently receiving treatment at East Surrey Hospital.
His colleague, a locally-based Constable, was unhurt.
A 16-year-old boy from Croydon was arrested on suspicion of assault with intent to resist arrest, possession of a large hunting knife, possession of cannabis, and attempted robbery, and is currently in custody for interview and further enquiries, according to police.
The other person involved, a 45-year-old Crawley man, was not injured or arrested and is being treated as a witness, added police.
Detective Sergeant Kirstie Neal said; “We are urgently appealing for any witnesses to this incident, and are particularly interested in speaking to four men who were seen walking past at the time.
“There is no suggestion that they were involved in any way but we believe they will have relevant information.
“If you can help in any way please contact us online or by calling 101, quoting serial 92 of 07/04.”