Hundreds of nationalist youths last night attacked a police station in Co Armagh.
Trouble flared after Armagh's Gaelic footballers clinched an historic victory in the All-Ireland final.
Security cameras were pulled down from the perimeter of the building in Lurgan. Tricolour flags were then put up on a war memorial in the nearby town centre, a Police Service of Northern Ireland spokesman said.
Around 30 rival loyalists who emerged from the Market Street area began fighting with the Catholic crowds.
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At least one police officer suffered minor injuries as security forces tried to end the fighting.
The PSNI said police trying to keep the crowds apart came under attack from the nationalist crowd throwing bottles and stones.
PA