There has been a tense stand-off in the Short Strand area of Belfast where loyalist and nationalist community workers have been trying to restore calm after riots left 29 police officers injured.
Loyalists returning from a demonstration in the city centre clashed with nationalists in the Short Strand area.
Reports suggest that loyalists were shepherded by police past the nationalist Short Strand area.
Minutes earlier, the marchers expected route over Queen's Bridge had been blocked by the PSNI's Land Rovers.
As the crowds passed the Short Strand, rival factions exchanged bottles, bricks, golf balls and other missiles.
The PSNI struggled to contain the situation and at one stage hundreds of loyalists were directed by the PSNI past Belfast's main central station and motorists got caught up in the mayhem.
Northern Ireland's first and deputy first ministers are to meet the Tánaiste next week as protests over the Union flag issue shows no sign of abating.
Tánaiste Eamon Gilmore has expressed concern at the further attacks on police officers last night and the ongoing disruption to civic life in Northern Ireland.
"This violence is being orchestrated and those behind it are known criminals, intent on creating chaos," he said in a statement.
"This has nothing to do with real issues around flags and identity in a shared society which are the subject of intensive political discussions at present."
It is 40 days since Belfast City Council made its controversial decision to reduce the number of days the Union flag is raised over city hall.
Last night's demonstrations showed there is no sign of an end to the anger in some loyalist communities.
The Stormont Assembly is due to re-open next week.
There has been ongoing contact between the administrations in London, Dublin and Belfast.
The meeting planned for next week between Peter Robinson, Martin McGuinness, Northern Ireland Secretary Theresa Villiers and Mr Gilmore is an effort to publicly show that the commitment to the progress made in recent years remains rock solid.
The Union flag controversy and the unrest that followed amount to the greatest test to date of the principles that underpin the Good Friday Agreement.
Police injured during riots
Four police officers were injured during riots last night.
Police fired plastic baton rounds and used water cannons during street disturbances in Carrickfergus, Co Antrim and in Rathcoole, on the northern outskirts of Belfast.
During the violence, police were attacked with petrol bombs, bricks and other missiles.
A number of vehicles, including a bus, were set on fire. Two people were arrested.
Northern Ireland Minister for Enterprise Arlene Foster said the trouble is damaging the image of the country.