Loyalist protesters threw Molotov cocktails and stones at police officers in Carrickfergus, an ancient town on the north shore of Belfast Lough, and in the Rathcoole neighborhood in Newtownabbey, the Belfast Telegraph reported. Police used water cannon to get the crowds under control.
Four officers were injured, including one who was hospitalized, police said.
Violent demonstrations began in early December when the Belfast council voted to limit the flying of the Union Jack at city hall to 17 days a year. The move was supported by nationalist parties and by the Alliance Party, a group that has attempted to bridge the sectarian divide.
The Progressive Unionist Party and the Democratic Unionist Party have both filed court challenges to the new flag rules. At the same time, their leaders have urged protesters to abandon violence and seek political and legal solutions.
Police said demonstrations were held around Northern Ireland on Friday but most were peaceful. In Belfast, some roads were blocked, hampering Rugby football fans trying to get to the Heineken Cup match between Belfast and Glasgow.
In the center city, an online campaign, Operation Sit-in, appeared to have been successful in bringing business to restaurants, pubs and cafes hit hard by the demonstrations, the Telegraph said.