The demonstrations which aimed to restrict movement throughout the region took place between 6pm and 8pm on Monday.
Six men and two women were arrested on suspicion of obstructing a highway following a protest in the Knock Dual Carriageway area of east Belfast.
Meanwhile in Londonderry's Waterside, three men were arrested on suspicion of a number of offences including disorderly behaviour and obstructing a highway.
In Belfast, Shankill Road at Browns Square, Cambrai Street, and Castlereagh Street were blocked by protestors.
Elsewhere police reported flag protests at Ballyclare Square, on Coast Road in Glenarm, on Rossdowney Road in Londonderry and in Fountain Hill in Antrim.
The Lisburn Road from Elmwood Avenue to Shaftesbury Square and the Albertbridge Road from its junction with Castlereagh Street and Mountpottinger Road to Connswater were also affected by protests.
The Shore Road at Mount Vernon, My Lady's Road, Malone Road at Dub Lane and Sandy Row at Boyne Bridge have reopened after roads blocks were lifted.
Meanwhile Translink said all east Belfast Metro services were suspended for a time "due to numerous protests" on Monday night.
The demonstrations began last month, after Belfast City Council voted to restrict the flying of the Union flag to 18 designated days.
Earlier this month the first so-called Operation Standstill took place as protestors took to the streets to voice their opposition to the Belfast flag decision across Northern Ireland.
Although violence has broken out at some flag protests, police said the majority of demonstrations were peaceful.