Politicians have been threatened during a council meeting in Northern Ireland when a small group of loyalists stormed the building.
There were no injuries but the session was disrupted in Carrickfergus, north of Belfast on the Co Antrim coast, police said.
The PSNI said about 80 protests took place across the province, with numbers ranging from small groups to hundreds of people. While most of the protests were peaceful, there was disorder in a number of areas including South and East Belfast, Portadown in Co Armagh and Carrickfergus. Police came under attack in a number of locations and one police officer has also been injured at this stage. There have been 15 arrests with more to follow as police continue with their robust investigations.
Earlier this month an Alliance party office in Carrickfergus was burned and there have been a series of death threats made against elected representatives in a row over a reduction in the number of days the Union flag is flown from Belfast city hall.
Dozens of Police Service of Northern Ireland members have been injured by violence which erupted earlier this month. Appeals for the protests to stop have fallen on deaf ears and Belfast traders have warned of a dismal Christmas with the streets relatively empty of shoppers.
Police were attacked by missile-throwing loyalists during the violence in south and east Belfast on Monday night. At one stage the PSNI was confronted by a crowd of 200, since dispersed.
Countless roads have been blocked across the city, causing disruption to shoppers and commuters days before Christmas. Some retailers said their businesses had been devastated by a campaign which has continued since the start of the month.
The Police Service of Northern Ireland said there had been demonstrations in Greater Belfast on Monday night, including the neighbouring towns of Lisburn and Carrickfergus.
First Minister Peter Robinson and Deputy First Minister Martin McGuinness have appealed for the protests to stop. And on Monday, Northern Ireland Secretary Theresa Villiers met leaders of Northern Ireland political parties to discuss the flags controversy. "I have today urged Northern Ireland's political leaders to come together to find a way forward to resolve the ongoing protests on flags," she said. "It is vital that all the parties work together to make progress towards a cohesive and shared society where decisions on issues like flags can be made in a way which respects different views and takes into account the different traditions and identities of modern Northern Ireland."
Cross-community Alliance Party councillor Noel Williams said about five protesters infiltrated a meeting of Carrickfergus Council. "They managed to enter the council chamber unhindered without facing any opposition and once inside subjected councillors to verbal abuse, banging on desks and chairs with implements, leaving many feeling threatened," he said. "It is unacceptable that the town hall was not under police surveillance, especially as violence has broken out at previous protests."