Seamus Mallon, an architect of the Northern Ireland peace process who served as deputy first minister, has died at the age of 83.

Tributes from across the political spectrum in Ireland and the UK poured in after the SDLP announced on Friday that its former deputy leader had died.

Mallon was a nationalist who desired Irish unity but was a scathing critic of republican violence.

He described the 1998 Good Friday agreement as “Sunningdale for slow learners” – a withering rebuke to extremists on both sides who brought down the 1973 Sunningdale agreement and extended the Troubles, costing thousands of lives.

“Seamus Mallon was a force of nature,” Colum Eastwood, the SDLP’s leader, said in a statement. “In the darkest days of conflict, when hope was in short supply, Seamus represented the fierce thirst for justice that ran through the SDLP and through communities that had lost so much to political violence.”

Eastwood, whose election as an MP last month marked a revival of the party’s fortunes, said Mallon’s passion to end the Troubles underpinned by truth, justice and reconciliation helped lead Northern Ireland to peace. “It didn’t matter who you were, where you worshipped or what your politics were, there was always help to be found at Seamus’s hearth.”

Ireland’s president, Michael D Higgins, said Mallon possessed unsurpassed courage, civility and fairness. “He was instrumental in bringing into being a meaningful discourse that heralded a new possibility of civil rights within a shared island.”

The taoiseach, Leo Varadkar, said: “History will remember Seamus as an architect of the Good Friday agreement, a committed peace builder and a tireless champion of an inclusive Ireland.”

The former US president Bill Clinton, who was a key player in the Good Friday agreement, hailed Mallon as a “hero of the peace process in Northern Ireland” and “a profoundly good man”. He said: “As his party’s chief negotiator in the talks leading to the Good Friday agreement, he was respected by all parties for his intelligence and integrity, his candour and convictions.”

Clinton added: “I will always be grateful for the opportunity to have known and worked closely with him. My thoughts and prayers are with his family and all the people whose lives he touched.”

Lord Empey, the Ulster Unionist peer, lauded Mallon’s sense of humour and principles. “I don’t believe the Good Friday process could have succeeded without him. He understood the practicalities and realities of politics and government, something that some of his colleagues failed to appreciate ... I think all of us have lost a champion of democracy and justice today.”

Mary Lou McDonald, the Sinn Fein leader, issued a short statement expressing condolences to Mallon’s family and recalling his contribution to Irish politics and the Good Friday agreement.

Born in Markethill, County Armagh, Mallon was a school principal who entered politics via the civil rights movement in the 1960s.

As deputy leader of the SDLP from 1979 to 2001 he served – and often clashed with – the party’s mercurial, towering leader, John Hume.

After the Good Friday agreement Mallon served until 2003 in the power-sharing administration at Stormont with the unionist first minister David Trimble, another fraught but effective partnership.

Last year with the journalist Andy Pollak he published a book, A Shared Home Place, which warned against a united Ireland unless and until at least a substantial minority of Northern Ireland’s Protestants were in favour – a rebuke to nationalists who seek a unity referendum decided by simple majority.