Chief Constable Simon Byrne has said heavily armed police are needed to patrol some parts of Northern Ireland after he posted a photo on Twitter of himself with officers in County Armagh.
He joined Police Service of Northern Ireland (PSNI) colleagues working in Crossmaglen on Christmas Day.
To mark the visit, he posed for a picture with officers holding rifles.
Some nationalist politicians criticised him, saying the image would undermine confidence in policing in the area.
In a message accompanying the photograph, Mr Byrne said the "sense of duty and optimism" of the officers in Crossmaglen was inspiring.
He described the area as a "unique part" of the PSNI patrol.
Conor Murphy, a Sinn Féin MLA for the area, said the post was "offensive to the local community and utterly unacceptable".
'Questions must be asked'
SDLP councillor Pete Byrne said the wording of the post "appears to be a deliberate attempt to set Crossmaglen and south Armagh as a place apart".
"A lot of work has been done in recent years to build community relations with the PSNI locally," he added.
"While we all appreciate police officers... working on Christmas Day, questions must be asked about the nature of this post.
"Language and images like this flies in the face of the positive work that has been achieved."
In response to the criticism, Mr Byrne tweeted on Thursday to say he had seen his Christmas Day post had "caused some emotion".
But he added: "The officers are carrying weapons and sadly this is the stark reality of the policing environment they work in, tackling the severe terrorist threat."
He said the officers in Crossmaglen "work in unique and often difficult circumstances".
"This is no reflection on the local community who work daily with their local police officers to create a safe community."