Gerry Fitt, 79, a leader of Catholic nationalists in Northern Ireland and a fierce critic of the Irish Republican Army, died Aug. 26 in England, the family said.
The cause of death was not announced, but he had a history of heart disease and had been in declining health for months.
Mr. Fitt was a leader of a Catholic civil rights march that confronted police near Londonderry on Oct. 5, 1968. Violence broke out, gaining worldwide television coverage, and it effectively marked the start of three decades of violent "troubles" in Northern Ireland.
Mr. Fitt, a member of Britain's House of Commons, was one of the first to be clubbed by police. He recalled saying a prayer of thanks as he felt blood flow down his face.
"I knew that, at last, Northern Ireland as she really was would be seen before the world," he said.
A former merchant seaman, Mr. Fitt represented West Belfast in the House of Commons for 17 years and was one of the founders in 1970 of the Social Democratic and Labour Party, which attracted the votes of moderate Catholics.
In 1974, he served as deputy chief executive of a power-sharing government that collapsed within five months because of Protestant opposition.
In 1976, armed with a pistol, he faced down a mob of IRA supporters who had invaded his home in Belfast.
"The thoughts that went through my head at the time, all in the space of a few seconds, were: 'Gerry, this is how you die'; secondly, 'I hope to God you don't get my wife and young daughter'; and thirdly, 'I hope to God I don't have to pull this trigger and kill someone myself.' "
His home was later burned down, and he moved his family to London.
Mr. Fitt left the SDLP in 1979, criticizing its increasing insistence on seeking greater involvement by the Republic of Ireland in Northern Ireland's affairs -- the line taken by his successor as party leader, John Hume.
Mr. Fitt, standing as a Socialist candidate, lost his Commons seat to Gerry Adams, leader of the IRA-allied Sinn Fein party, in 1983, and subsequently was appointed to the House of Lords as Baron Fitt of Bell's Hill in the county of Down.
Mr. Fitt's wife, Ann, died in 1996.
Survivors include five daughters.