Ivan Cooper, who has died aged 75, was an exceptional figure in the tempestuous politics of Northern Ireland, and the person who organised the 1972 civil rights march in Derry that ended in what became known as Bloody Sunday.

After the introduction of internment without trial, the Unionist-controlled Stormont government banned marches in an attempt to tackle months of protest and public disorder in Derry. Cooper ignored the ban, and was one of the campaign leaders at the head of the illegal march in Derry on 30 January 1972, when members of the 1st Battalion Parachute Regiment opened fire on the crowd, shooting 14 people dead and wounding many others. The catastrophic consequences outraged the Catholic community, increased support for the IRA and destroyed the prospect of any imminent political initiative.

There was a surge of murder and violence in all parts of Northern Ireland in subsequent years. In the 2002 film Bloody Sunday, Cooper (played by James Nesbitt) was portrayed as saying: “I want to say this to the British government ... You know what you’ve just done, don’t you? You’ve destroyed the civil rights movement, and you’ve given the IRA the biggest victory it will ever have. All over this city tonight, young men, boys, will be joining the IRA and you will reap a whirlwind.”

Bloody Sunday was also a personal watershed for Cooper. Born into a Unionist (and Protestant) family, he had abandoned his youthful allegiance to Unionism to become a leader of the cross-community Northern Ireland civil rights campaign and then a founding member of the nationalist (Catholic) Social Democratic and Labour party (SDLP) in 1970. As a result, for the rest of his life he was called a “traitor” by his adversaries, and while he continued to be loyal to the Anglican Church of Ireland, some worshippers refused to share a pew with him.

Ivan was born in the village of Claudy, Co Derry, and was brought up for a while in nearby Killaloo, where his parents, Jeannie (nee Moorehead) and Albert Cooper, ran the local post office, until the family moved to Derry in 1956. After school he worked in Derry as a factory line manager in the shirt-making business, the city’s staple industry. Moved by the poverty he saw among the workers, and by the anti-Catholic discrimination manifested by Derry’s Unionist-dominated council, his first move into politics came in 1965, when he briefly joined the Claudy Young Unionist Association.

Shortly afterwards he resigned, switching to membership of the Northern Ireland Labour party, which was trying to persuade working-class voters to join together on a non-sectarian basis. Cooper stood for election to the Stormont parliament in 1965, winning some cross-community support but failing to gain a seat.

Around that time there were widespread demands for the Unionist government to introduce reforms to benefit the entire working class on a non-sectarian basis. Cooper, by then resident in Derry, was at the centre of the campaign and in 1968 left the Labour party to become founder of the Derry Citizens’ Action Committee. His rallying call was for Catholics and Protestants to fight for their rights as “the blacks in America were fighting”.

Later that year the committee announced it would organise a parade in Derry to demonstrate the great support there was for reform. The Unionist minister for home affairs, William Craig, slapped a ban on the parade and drafted in hundreds of police with batons and a water cannon. There followed serious clashes as the police tried without success to enforce the ban. The incident is now remembered as the start of Northern Ireland’s Troubles.

Cooper, who was opposed to all violence, increasingly found himself on the street trying to ease tension and prevent demonstrators from coming into contact with the police and Unionist factions. His fellow civil rights activist Paddy Devlin admired the way his outdoor oratory could reach to even the largest crowds, but on more than one occasion he was knocked out cold by bricks and stones.

He was first elected, as an independent representative, to the Stormont parliament in 1969, the first of many electoral victories. In 1970 he co-founded the SDLP along with Devlin, John Hume, Austin Currie and Gerry Fitt, and the high water mark of his political career came in 1973 when he was elected to the power-sharing executive at Stormont, where he gained ministerial responsibility for community relations. But the executive was jinxed, foundering after only five months in office when the United Ulster Unionist Coalition brought it down with a general strike.

Cooper and the SDLP continued to fight elections and to pursue the elusive goal of power-sharing, cross-community co-operation and an end to the Troubles. In 1983, with great disappointment, he withdrew from active politics and became an insolvency practitioner. After a stroke, his health progressively deteriorated and he became a wheelchair user. Nonetheless, much of the work that Cooper had done helped ultimately to smooth the way for the Good Friday Agreement in 1998.

Over the years, the events of Bloody Sunday became mired in a propaganda war over culpability that continued until the Saville Inquiry finally concluded in 2010 that there “was a serious and widespread loss of fire discipline among the soldiers”. Even though Cooper’s health had declined, he had strongly supported families of the bereaved in their campaign for truth and justice and continued to give pin-sharp interviews on the subject. He was present with the relatives when the Saville Report was published, and although the prime minister, David Cameron, made a formal apology and described the paratrooper operation as “unjustified and unjustifiable”, Cooper called for the soldiers concerned to be prosecuted. “In my church the commandment says ‘thou shalt do no murder’, and I believe that commandment,” he said.

He is survived by his wife, Frances, and their daughters, Bronagh and Sinéad.

• Ivan Averill Cooper, civil rights campaigner, born 5 January 1944; died 26 June 2019