SDLP leader Mark Durkan said he was "deeply saddened" by the news of the death of Lord Fitt, who he described as a key figure in the civil rights movement and politics.
"He was instrumental in founding the SDLP on the principles of non-violence, partnership and equality and in bringing about the Sunningdale Agreement, with its core features of power-sharing, a strong all-Ireland dimension and human rights.
"The tragedy for him and everyone else was that Sunningdale was opposed and brought down by intransigent unionism and violent republicanism — the same people who now claim they are for the principles that were at its core."
Ulster Unionist Lord Maginnis said he had a high regard for Lord Fitt. "Gerry was a genuine socialist - a man who believed and grew to believe even more strongly in his latter years in fair play for everybody," he said. "He was a man who despised terrorism and opposed terrorism and was exemplary in the way he courageously stood up to those in republicanism involved in terrorism."
Democrat Unionist leader, the Rev Ian Paisley, one of Lord Fitt's political opponents during the 1970s, also expressed his sorrow. "I am very sorry to hear of the passing of Gerry Fitt," the North Antrim MP said. "I offer my sincere sympathies to his family circle and his friends at this time."
Alliance Party leader David Ford said he was one of the few Northern Ireland politicians who could attract votes right across the community. "In the power-sharing executive of 1974 his role was crucial in helping to bring people together."
Taoiseach Bertie Ahern said Lord Fitt gave strong leadership to all communities during troubled times in Northern Ireland.
"During a long and brave career in the wider Labour movement and as a public representative, he made a very significant contribution to constitutional politics and civil rights in Northern Ireland.
"He was a man who practised the message of moderation and tolerance, that he also courageously preached.
"He was often in the front line of the Troubles and he experienced violence at first hand from both sides of the divide in the North.
"Above all he abhorred sectarianism and violence.
"In the ongoing quest for a peaceful settlement and constitutional politics in the North, history will record that he played his part by word, by deed and by example."
President Mary McAleese said Lord Fitt made an enormous contribution to Northern Ireland politics and would be sadly missed by colleagues and friends.
"He was a committed lifelong socialist and a Belfast man through and through. "Over the years he had so many friends and admirers from across the entire spectrum of politics," she said today.
Labour Party leader Pat Rabbitte claimed Lord Fitt was one of the most important figures in Irish politics during the last 50 years. He said the former SDLP leader put Labour politics on the map in Northern Ireland through his activism at grassroots level and election to Belfast City Council, Stormont and Westminster.
"He was always a strong and passionate advocate of civil rights at a time when unionists failed to recognise or respond to these calls," said Mr Rabbitte. "He also demonstrated great personal courage in the face of worsening sectarianism."
Fine Gael leader Enda Kenny said the former SDLP leader was "truly remarkable politician" who made a huge contribution to Northern Ireland.
"Throughout his career, Gerry Fitt demonstrated huge courage, both in his steadfast opposition to all paramilitary violence and in exposing the criminality in which the provisional movement was engaged," he added.