Bollards have been erected outside Strand Road police station in Londonderry to protect the building from attack by dissident republicans.
Parking spaces adjacent to the station have also been removed, and replaced with a steel barricade.
District Commander Chief Superintendent Stephen Martin said it was in response to a "severe threat".
He also said it was "the correct, necessary and proportionate thing to do".
"We understand the inconvenience this may cause however the safety of the public, police officers and police staff must be the priority.
"We have talked to a number of people including local residents, the Chamber of Commerce and the City Centre Initiative to explain our reasons for taking this step, and the matter will be kept under review."
It follows an increase in security after a bomb exploded outside the police station in August.