Former US president Bill Clinton has appealed to all sides involved in Northern Ireland's peace process to "finish the job".
In a speech to an audience in Derry city centre's Guildhall Square, he cited the example of Nelson Mandela, who lived in the present for the future and who forgave his jailers to set himself free.
Mr Clinton warmly praised former SDLP leader and Nobel Laureate John Hume for his role in promoting peace and reconciliation in Northern Ireland.
He commented that no one should underestimate the impact the peace process has had on the entire world.
He said that from Colombia to Indonesia and from the Basque country to Burma, events in Ireland inspire.
The former president began his day-long visit to Northern Ireland by crossing a symbolic peace bridge.
Mr Clinton was accompanied by Mr Hume and his wife Pat as he walked along the footbridge linking divided communities in Derry.
On his fifth visit to Derry, Mr Clinton also helped publicise a book on peacemaking produced by the University of Ulster.
Tonight in Belfast, he officially opened a leadership institute named after him at Queen's University.
Before delivering the inaugural William J Clinton leadership lecture at Queen's, he met Northern Ireland's First Minister Peter Robinson and Deputy First Minister Martin McGuinness at Stormont to discuss current challenges facing the power-sharing institutions.
The 67-year-old was heavily involved in the peace process when he was president, especially in the run-up to the signing of the Good Friday Agreement in 1998.
He first visited Belfast in 1995.
Mr Hume said the visit of the former US president was an "incredible honour" for his city.
"I have known Bill Clinton for 22 years and I have met him every time I travelled to Washington, and I have always had the greatest admiration for him," he said.
"I am deeply appreciative for all the work he has done to help Northern Ireland, in spite of all the difficulties during his time in the Oval Office.
"Bill Clinton had economic difficulties and international difficulties to deal with during his administration, yet he gave so much time to Northern Ireland and the peace process."
Yesterday, Mr Clinton paid a courtesy call to Taoiseach Enda Kenny at Government Buildings.