LONDON (AFP) - Northern Ireland's First Minister Peter Robinson on Sunday called for peace as riots erupted in Belfast for the third night running, injuring a police officer.

Seven officers were injured on Saturday after being attacked with petrol bombs by Protestant rioters in the Northern Irish capital, a spokeswoman said.

"It's very important that this violence stops," said Mr Robinson.

"It's very important that cool heads prevail in these circumstances and I hope people will obey the announcement and statement by the Orange Institution that people should desist from violence.

"The only kind of protest that is ever justifiable is a lawful and peaceful protest," he added.

The Police Service of Northern Ireland (PSNI) said one officer had been injured in Sunday's clashes.

Bricks, bottles, furniture and other missiles were hurled on Saturday night by hooded youths, some with British flags covering their faces.

The seven officers injured on Saturday did not require hospital treatment and remained on duty.

On Friday night, 32 officers were injured and a leading politician was knocked unconscious by a brick.

About a thousand police officers from mainland Britain were sent to Northern Ireland in anticipation of tensions over the traditional Twelfth of July parades, the pinnacle of the Protestant Orange Order's marching season.

Trouble flared Friday after police tried to enforce a decision by an adjudication body banning the Orange Order from marching through a Catholic republican area of Belfast.

"The scenes were both shameful and disgraceful," Chief Constable Matt Baggott of the PSNI told reporters on Saturday.

He criticised leaders in the Orange Order who had called for protests against the decision to block their march through the republican Ardoyne area, saying they had been "reckless".

Nigel Dodds, the member of parliament for North Belfast, was taken to hospital after being hit on the head with a brick while trying to calm the crowds down on Friday night. He was later discharged.

The July 12 parade marks the victory of Protestant king William III of Orange over the deposed Catholic king James II at the Battle of the Boyne in 1690.

It is a flashpoint for tensions between the Protestant and Catholic communities in the province, which was devastated by three decades of sectarian violence in the 1970s, 1980s and 1990s.

The 1998 Good Friday peace accords largely brought an end to the unrest, known as The Troubles, although sporadic violence and bomb threats continue.