PSNI chief constable Simon Byrne has been criticised for posing alongside officers armed with machine guns outside a police station in Crossmaglen.
His Christmas Day tweet pictured him beside the fortified barracks in south Co Armagh.
Sinn Féin Assembly member Conor Murphy demanded "immediate answers".
He said: "This post by the chief constable is offensive to the local community and utterly unacceptable.
"It has only served to further undermine the public confidence in the PSNI within South Armagh.
"The heavily armed officers posing with the chief constable is reflective of the militaristic style of policing that the community in South Armagh has had to endure in recent years."
In recent times Mr Byrne has warned of the opportunities a hard Irish border might provide for dissident republicans.
He visited the border police station and acknowledged the unique nature of policing in the area.
On Christmas morning great to meet the team policing Crossmaglen @PSNINMDown I take my hat off to colleagues policing such a unique part of @PoliceServiceNI Their sense of duty & optimism is inspiring. Stay safe and thank you . pic.twitter.com/2Bj4bV9d48— Simon Byrne (@ChiefConPSNI) December 25, 2019
Mr Murphy said: "We need a new chapter for policing in South Armagh.
"We need an end to fortress police stations, heavily armed officers and poor response levels.
"We need a police service that serves the interests of this community in an efficient, effective and respectful manner.
"This community not only deserves nothing less, but will accept nothing less."
Mr Byrne applauded colleagues who police such a unique part of Northern Ireland.
He said: "Their sense of duty and optimism is inspiring. Stay safe and thank you."
A PSNI statement said: "The chief constable is happy to meet with any community or political representative who has a positive contribution to make about local policing matters."