Operation Demetrius was a British Army operation in Northern Ireland on 9-10 August 1971, during The Troubles. It involved the mass arrest and internment (without trial) of 342 people suspected of being involved with Irish republican paramilitaries (the Provisional IRA and Official IRA). Armed soldiers launched dawn raids throughout Northern Ireland, sparking four days of rioting that killed 20 civilians, two IRA members and two British soldiers.
Of the civilians killed, 17 were killed by the British Army and the other three were killed by unknown attackers[About 7,000 people fled their homes, of which roughly 2,500 fled south of the border. The policy of internment was to last until December 1975. Its introduction, and the brutal treatment of those interned, led to numerous protests and a sharp increase in violence.
On the initial list of those to be arrested, which was drawn up by RUC Special Branch and MI5, there were 450 names, but only 350 of these rendered themselves available for internment. Key figures on the lists, and many who never appeared on them, were warned before the swoop began. It included leaders of the non-violent civil rights movement such as Ivan Barr and Michael Farrell. But, as Tim Pat Coogan noted,
What they did not include was a single Loyalist. Although the UVF had begun the killing and bombing, this organisation was left untouched, as were other violent Loyalist satellite organisations such as Tara, the Shankill Defence Association and the Ulster Protestant Volunteers. It is known that Faulkner was urged by the British to include a few Protestants in the trawl but he refused.
In terms of loss of life, 1972 was the most violent of the entire Troubles. The fatal march on Bloody Sunday (30 January 1972) in Derry, when 14 unarmed civil rights protestors were shot dead by British paratroopers, was an anti-internment march.