A former soldier is the first person arrested by Northern Ireland police in connection with the 1972 Bloody Sunday shootings that killed 14 civil rights protesters.
The 66-year-old former paratrooper from Country Antrim is being questioned at a police station in Belfast, in what the police described as a "new phase in the overall investigation" into the incident.
Fourteen people died as a result of the events of January 1972, where British paratroopers fired on unarmed protesters in the Bogside area of Derry. Thousands of people had been marching against the government's policy of holding suspected members of the IRA without trial.
The incident became one of the most controversial and symbolic of the Troubles, after an initial UK government investigation cleared the soldiers of wrongdoing.
As part of the Northern Ireland peace process, a public inquiry was established under Lord Saville in 1998 to provide a definitive account of the shootings. Following more than a decade of work, Saville concluded that the army had fired first and the protesters had been killed unlawfully, prompting the Police Service of Northern Ireland to launch a murder inquiry into the deaths.
In 2010 David Cameron apologised for the incident on behalf of the British state: "Some members of our armed forces acted wrongly. The government is ultimately responsible for the conduct of the armed forces and for that, on behalf of the government, indeed, on behalf of our country, I am deeply sorry."
"Detectives from Legacy Investigation Branch investigating the events of Bloody Sunday have arrested a 66-year-old man in County Antrim," the police said in a statement.
"The suspect is being interviewed at a police station in Belfast. The officer leading the investigation, Detective Chief Inspector Ian Harrison, said today's arrest marked a new phase in the overall investigation which would continue for some time."