The Police Service of Northern Ireland (PSNI) have fired plastic bullets and water cannons at protesters late on Monday during a fifth consecutive night of riots during flag protests in the capital Belfast.
The PSNI were bombarded with weapons, including hatchets and petrol bombs, which were used to attack police officers and their vehicles in east Belfast.
Street demonstrations have been held in Belfast almost every night since 3 December, when the city council announced that it would no longer fly the British flag all year round at the City Hall.
They will only fly the union flag on 17 designated days, including the birthdays of members of the British royal family - the first of which falls on Wednesday, with the birthday of Prince William's wife Catherine.
The decision sparked riots at the start of December which gave way to largely peaceful protests, but the violence has flared again since the start of the new year.
Ireland's deputy prime minister Eamon Gilmore called for an end to the violent protests. "These attacks over the past four days are not the mark of legitimate protest but are the actions of a small group who want to bring Northern Ireland back to a darker past," Gilmore said.
Britain's Northern Ireland minister Theresa Villiers said the province was being "held to ransom" by the protesters and called for an end to their demonstrations, including peaceful rallies that have blocked traffic for weeks.
"It's not acceptable that those who say they are defending a Union flag are actually doing it by hurling bricks and petrol bombs at police. It's disgraceful, frankly," she told BBC radio.
She added that the protests were doing "huge damage to Northern Ireland's image abroad."
The flag ruling has raised tensions between loyalists who want to maintain the links to Britain and are mostly Protestant and largely Catholic republicans who want a united Ireland.
PSNI Chief Constable, Matt Baggott, said individual senior loyalist paramilitaries had been involved in orchestrating violence during union flag protests in east Belfast.
Amid deepening tensions in the province, politicians from both sides have received death threats in recent weeks, the National Union of Journalists has confirmed that a reporter in Belfast has been sent a bullet in the post along with a threatening letter.
More than 60 police officers have been injured and over 100 people arrested since the disorder began at the start of December.
The PSNI said four people had been charged in connection with Monday night's disorder and were due to appear in court on Tuesday.
In 1988 a peace agreement brought an end to most of the violence and led to the creation of a power-sharing government between Protestants and Catholics, but bomb threats and murders by dissident republicans still continue.
Loyalists see the council's decision to remove the flag for most of the year as an attack on their British identity and an unacceptable concession to republicans.
The last major loyalist demonstration in the Irish capital sparked rioting and looting in 2006.