Belfast, Northern Ireland (CNN) -- Under a gray, overcast sky, more than 1,000 protesters gathered Saturday in the Northern Ireland city of Belfast carrying large Union flags, some wrapped around their shoulders.
The silent protest outside the city hall started out peaceful, the quiet only broken by the noise of helicopters circling overhead, conducting surveillance. A few hours later, Protestant rioters got into skirmishes with police. As the protesters passed a Catholic neighborhood, Protestants and Catholics threw bottles at each other.
Many police were in the area, accompanied by dozens of heavy armored vehicles.
The rally, followed by a march to East Belfast, is the latest in a wave of protests prompted by Belfast City Council's December 3 decision to limit the flying of the Union flag to 18 days per year.
Protests were also held across Northern Ireland on Friday night.
Most were peaceful, police said, though trouble broke out in places.
One of the most serious incidents occurred at O'Neil's Road in Newtownabbey, where about 100 Protestant rioters -- many of them teenagers -- threw concrete blocks, paving slabs, bricks, planks of wood, gasoline bombs and Molotov cocktails at police.
They targeted about 30 armored police vehicles and as many as 100 police officers clad in riot gear. In all, four police officers were hurt, with one requiring hospital treatment, and two people were arrested, police said.
Amid rain and temperatures just above freezing, police brought in two water cannon, deploying one of them to push back the rioters.
Temperatures are also expected to drop later Saturday as the protesters march to East Belfast, a trouble hotspot in recent weeks.
The city was rocked by five consecutive nights of rioting at the start of the week.
The leaders of the two main unionist political parties met Thursday to discuss concerns within the unionist community.
The flag was raised Wednesday for the first time since the controversial vote to mark the birthday of Prince William's wife, Catherine. It was lowered at the end of the day.
Northern Ireland authorities have accused loyalist extremists of exploiting the decision by Belfast officials to end the century-old tradition of flying the Union flag over City Hall 365 days a year.
The British flag has long been a flashpoint between British loyalists and Irish nationalists, who want Northern Ireland to join the Irish republic.
The Belfast City Council vote followed a summer of heightened tensions between Northern Ireland's Catholic and Protestant communities. Riots in September left dozens of police injured.
The majority of Ireland gained independence in 1921 following two years of conflict. But six of the nine counties of the province of Ulster chose to stay in the United Kingdom, eventually becoming Northern Ireland.
In the late 1960s, the conflict between mainly Protestant loyalists, who want Northern Ireland to remain part of the United Kingdom, and largely Roman Catholic nationalists, who want it to be reunited with the rest of Ireland, exploded into a political and sectarian war, known as "the Troubles."
The three decades of ensuing violence between loyalists and the IRA claimed more than 3,000 lives, most of them north of the border. While the Good Friday Agreement of 1998, also known as the Belfast Agreement, effectively ended the conflict, distrust remains between Catholics and Protestants.
Under the terms of the accord, groups on both sides dumped their weapons, and members of Sinn Fein, the political affiliate of the IRA, now work with pro-British politicians in Northern Ireland's power-sharing government.
The Union flag is also commonly known as the Union Jack. Some say the term jack should only be used to refer to the flag when it's flown on a warship but according to the Flag Institute, which runs the UK flag registry, this is a relatively recent idea.