A new U.K. -EU trade deal has imposed customs and border checks on some goods moving between Northern Ireland and the rest of the U.K. The arrangement was created to avoid checks between Northern Ireland and Ireland, an European Union member, because an open Irish border has helped underpin the peace process built on the 1998 Good Friday accord.
Police have come under attack in County Antrim as violence flared during another night of sporadic disorder.
Northern Ireland police said today that Unionist protesters threw 30 petrol bombs at officers and torched cars during a night of "disorder" in a suburb of northern Belfast. The decision is being reviewed.
Cars were set alight and masked people pelted a police van with petrol bombs in further disorder in pro-British parts of Northern Ireland on Saturday amid rising post-Brexit tensions in the region.
In Co Antrim, a recent series of drug seizures against the South East Antrim UDA - a renegade faction of the main grouping - have caused particular ill-feeling towards police.
"I condemn the violence, I condemn those that orchestrate it, and I understand the sadness it is that there are some as young as 13 and 14 that were arrested for being involved in that".
The faction is believed to have been behind some of the weekend disturbances.
District commander, Chief Superintendent Simon Walls, said officers had suffered burns, head and leg injuries after being subjected to "a sustained attack" by rioters throwing heavy masonry, metal rods, fireworks and manhole covers.
All the main unionist parties have demanded the resignation of Northern Ireland's police chief over the controversy, claiming he has lost the confidence of their community.
"To those in our community with influence, I would urge you to use that influence so we do not see any further disgraceful scenes of violence on our streets so we can ensure young people do not get caught up in criminality and that they are kept safe and away from harm".
Sinn Fein MLA Gerry Kelly claimed the disturbances were "a direct outcome of the actions of political unionism".
The disturbances broke out in the loyalist O'Neill/Doagh Road area of Newtownabbey, on the outskirts of Northern Irish capital, this past Saturday evening.
DUP leader Arlene Foster and other unionist parties have condemned the violence.
"By their words and actions they have sent a very risky message to young people in loyalist areas".
Four adults - three men, aged 25, 21 and 18 years old, and a woman, aged 19 - will appear at Belfast Magistrates' Court on 30 April.
Three teenagers, aged 17, 14 and 13, have also been charged with riot.
An eighth person arrested in connection with the unrest on Friday night, a man aged 19, has been released on police bail pending further inquiries.