Police used water cannon and fired a plastic bullet during the clashes in Belfast, the latest to blight the British province after more than five weeks of violent disorder over the flag issue.

"Four officers have now been injured during disorder in the Castlereagh Street of east Belfast this afternoon," a police spokesman said.

"Water cannon has now been deployed. Police have fired one AEP round", the spokesman added, referring to so-called baton rounds, or rubber bullets.

Witnesses said rocks and fireworks were thrown as lines of police tried to keep loyalists the Protestant community's working-class hardcore apart from Catholic nationalists in the Short Strand area of Belfast.

Nearly 1,000 people earlier gathered outside Belfast City Hall to protest.

Northern Ireland has been swept with a wave of sometimes violent protests since December 3, when Belfast City Council voted to restrict the number of days the British flag is flown at City Hall to 18 per year.

Loyalists see the council's decision to restrict the flying of the flag as an attack on their identity and an unacceptable concession to republicans seeking a united Ireland.

A 1998 peace agreement brought an end to the three decades of sectarian violence between Protestants and Catholics known as the Troubles, but sporadic bomb threats and murders by dissident republicans continue.