BELFAST (Reuters) - At least eight police officers were injured in Northern Ireland overnight as riots sparked by a decision to remove the British flag from Belfast City Hall spread across the province.

Dozens of youths threw bricks and petrol bombs at police in Newtownabbey, a predominantly Protestant area 12 kilometers (8 miles) north of Belfast city center. Seven people were arrested, including a 13-year-old boy, police said.

They were protesting against a decision by Irish nationalist councilors from Sinn Fein and the SDLP to take down the flag which had flown above the provincial capital's city hall every day since it opened in 1906.

The decision means the flag will be flown on only 17 days during the year, as is the case at the provincial assembly at Stormont in the British-controlled province.

Smaller protests took place across Northern Ireland on Friday during a visit by U.S. Secretary of State Hillary Clinton. She said the violence showed the province's peace process was not yet complete.

At least 3,600 people were killed over three decades as Catholic nationalists seeking union with Ireland fought British security forces and mainly Protestant Loyalists determined to remain part of the United Kingdom.

A 1998 peace accord has mostly held, although militant nationalists have stepped up attacks in recent years and community relations remain fragile, with riots erupting every few months at flashpoints in the city.

(Reporting by Ian Graham and Conor Humphries; Editing by Tom Pfeiffer)