Loyalists were urged tonight to cancel more planned street demonstrations over a Belfast City Hall decision to restrict the flying of the Union flag.
Amid fear of further violence following attacks on offices of the cross-community Alliance Party, Northern Ireland First Minister Peter Robinson called on protesters to suspend their action.
US Secretary of State Hillary Clinton is due to visit Northern Ireland tomorrow and is expected to endorse appeals for calm.
There were threats to hold another mass demonstration in Belfast city centre on Saturday, one of the busiest shopping days of the year, when many people could be at risk if violence breaks out, police said.
Police Service of Northern Ireland (PSNI) assistant chief constable Will Kerr warned: “To encourage thousands of people to come to Belfast city centre on one of the busiest days of the year would be madness.”
Mr Robinson said the action should be abandoned.
“My advice is that street protests should be suspended by those responsible for organising them in the wider interests of a peaceful society and to ensure their protests are not used by others to launch a campaign of violence,” he said.
“Britishness will not be progressed by acts of violence. Anyone engaging in wanton violence or intimidation does not defend our national flag but disgraces it.”
Anti-sectarian Alliance members of the regional assembly at Stormont and Westminster have been targeted at their homes and offices by arsonists, vandals and pickets.
It follows the party‘s pivotal backing for a reduction in the number of days which the flag can be flown from the city hall each year.
Trouble flared on Monday night after a protest outside the council meeting which ratified the decision, championed by some nationalists but bitterly opposed by many unionists, turned ugly. Golf balls, bottles and crush fencing were thrown at police officers.
Crowds breached the back gate of the city hall for the first time and police admitted today they were over-reliant on physical security.
Rioters surged into the courtyard close to where councillors were meeting but were repulsed by officers with batons and dogs. Eighteen people were hurt including police, security staff and a press photographer.
Police told today‘s meeting of the Northern Ireland Policing Board in Belfast they had taken a low-key approach to the protest, with only a handful of officers inside the building, but said there were people present who were intent on trouble.
Last night, unrest spread to a small number of other areas in Northern Ireland.
In Carrickfergus, Co Antrim, more than 1,000 rioters burned down the office of Alliance MLA Stewart Dickson and four police officers were injured as non-lethal rounds were fired to dispel protesters.
People from a loyalist paramilitary background were present but police said they were not actively encouraging violence, despite doing little to stop it. Two more teenagers were charged with affray tonight.
In the seaside town of Bangor, Co Down, arsonists last night attempted to set light to the office of Stormont Employment and Learning Minister Stephen Farry but were thwarted by a passing police patrol.
In the same town, a window was smashed at the home of two Alliance councillors with a 17-month old baby.
Councillor Christine Bower said: “It was extremely upsetting. I could not stop shaking, I just was beside myself.”
Chief Constable Matt Baggott said people were acting without thinking because of anger.
“That anger leads to problems, which leads to people losing their lives.”
He warned politicians not to engage in dogma over flag issues which could make matters worse, pointing to the tolerance shown by the Queen, and met Mr Robinson to ask for his help in cooling tensions.
“Loyalism can never be an excuse to compromise democracy, to use mob rule and violence as a way of asserting people’s will and compromising the rule of law, and I call upon people to take a step back. There is far too much at stake for the future and for the here and now,” he said.
Sinn Fein Deputy First Minister Martin McGuinness said: “This is an attack on the democratic decision taken in Belfast City Hall by those who are democratically elected to represent the people of this city.”
Ireland’s deputy prime minister, Tanaiste Eamon Gilmore, said violence against Alliance should stop: “Such violent attacks on a democratic party are an attack on democracy itself. They are reprehensible and have no place in a civilised society.”