British PM apologises for Northern Ireland's Bloody Sunday

British prime minister David Cameron apologised on Tuesday for the 1972 killings by British troops of 13 protesters on Northern Ireland's Bloody Sunday after a long-awaited report said those shot were unarmed.

Cameron told parliament the findings unequivocally showed there was no justification for the shooting of civilians during a civil rights march in the city of Londonderry.

"What happened should never, ever have happened... Some members of our armed forces acted wrongly," Cameron said.

"For that, on behalf of the government, and indeed our country, I am deeply sorry," he said.

A huge crowd gathered in Londonderry's Guildhall Square, the intended destination of the 1972 march, to watch Cameron's statement on a giant screen. At the point when he made the apology, the crowd cheered and applauded.

Bloody Sunday changed the course of Northern Ireland's political and sectarian violence that erupted in the late 1960s and came to be known as "The Troubles". The conflict pitted nationalists, mostly Catholics, who wanted to secede and become part of the Republic of Ireland, against unionists, mostly Protestants, who wanted to remain part of the United Kingdom.

On Sunday January 30, 1972, British troops opened fire during an unauthorised march in the Bogside, a staunchly nationalist area of Londonderry. They killed 13 people and wounded 14 others, one of whom died later. The victims were all unarmed Catholics.

The killings drove hundreds of new volunteers into the illegal Irish Republican Army (IRA) armed group, which stepped up its campaign of bombings and shootings. It was not until 1998 that a peace deal was brokered in Northern Ireland.

After Cameron spoke, emotional victims' relatives addressed the crowd in Londonderry. One of them tore up a copy of a discredited 1972 official report that had exonerated the troops.

"The great lie has been laid bare. The truth has been brought home at last," Mickey McKinney, whose brother Willie was among the dead, told the crowd to huge cheers.

The report says that Martin McGuinness, then a senior IRA commander and now deputy first minister of Northern Ireland in a power-sharing government of nationalists and unionists, was in the area and probably armed with a sub-machine gun on the day.

"We are sure that he did not engage in any activity that provided any of the soldiers with any justification for opening fire," the report said.

After it was made public, McGuinness denied he was armed that day and said that was a smear by "British agents".

The Saville report was 12 years in the making and the costliest in British legal history at close to £200 million ($293 million). Chaired by Lord Saville, a British judge, the inquiry took evidence from 2,500 people from 1998 to 2004.

An exhaustive account of Bloody Sunday that gives chilling details of each shooting, the report describes jumpy soldiers charging into hostile streets and going into a shooting frenzy that left 13 dead in less than 10 minutes.

"There was a serious and widespread loss of fire discipline among the soldiers," says the report, which describes harrowing scenes such as the shooting of one man as he was crawling away from the troops, and of another as he lay mortally wounded.

Critics of the Saville inquiry, particularly from the military and the unionist camp, fear that some families may try to use the report to have the soldiers prosecuted.

That would be controversial after many killers from all sides of the Northern Ireland conflict were freed from prison as part of the 1998 Good Friday peace agreement.