The North remains without a devolved government
Former US President Bill Clinton and Senator George Mitchell have received the Freedom of Belfast City for their role in the Good Friday Agreement.
Mr Mitchell is also a former US envoy to Northern Ireland.
Speaking at the ceremony in Ulster Hall, Mr Clinton said: "I will always be grateful that I came to Belfast when peace had been made but the city was still troubled.
"When wise and good and decent people had to actually make a decision to do the right thing, to be the right sort of person, to give children the right sort of future".
Lord Mayor of Belfast, Councillor Nuala McAllister, said: "I am absolutely delighted to be here tonight, marking such an important milestone in our history and recognising the very significant contribution made by President Clinton and Senator Mitchell in peacebuilding.
"Both men played an integral role in helping to bring peace to this part of the world, and have always shown great affection towards the people of Belfast and Northern Ireland.
"Along with many other prominent peacebuilders, President Clinton and Senator Mitchell helped to bring our communities together, and paved the way for a brighter future for generations to come."
Both men will be the 83rd and 84th recipients of the Freedom of the City of Belfast.
Tuesday marks 20 years since the agreement was signed in Belfast.
It was overwhelmingly approved by referendums north and south of the border and was the culmination of a lengthy peace process.
Political leaders and those key to the drafting of the deal have been meeting in the city.
On Monday, Mr Clinton gave a speech saying compromise is not a dirty word, a nod towards the current political leaders in Northern Ireland.
He said the 20th anniversary should be celebrated 'not for what happened but for what can happen.'
"The Irish peace was born out of weariness of children dying and of lost chances," he said.
"The further you get away from that, the easier it is to take the absence of bad for granted and to live in this purgatory we are in now.
"It is a big mistake."
Speaking at Queen's University in Belfast, Tánaiste Simon Coveney said: "We need to keep in mind what it was like before the Good Friday Agreement and to remember why we felt such joy and optimism two decades ago.
"After 20 years of peace - fragile and imperfect peace, but peace nonetheless - some memories have faded.
"It is all too easy to forget the extent and impact of the conflict on people here.
"The legacy of loss, injury, fear and violence has left deep scars. And tragically those wounds are not confined to my generation or the generations which came before.
"The traces of the conflict on our physical, social and psychological landscapes are still evident today - evident in segregated schools and communities, in peace walls, in areas of economic deprivation, and along our shared border.
"They surface from time to time in Belfast, in Dublin, in London, in Omagh, in Armagh, in every town and village and hinterland.
"And these traces pale away in comparison to the pain and loss experienced by the very many people who were most affected by the conflict.
"The victims, the survivors, the bereaved, the injured, the carers, the loved ones.
"If there is one particular area in which we have failed in 20 years to make sufficient progress - and I know that there is more than one - it is in what we often call 'the legacy of the past'.
"We’ve used the phrase so often that we possibly risk losing sight of what it means.
"Thousands upon thousands of individual and family stories. Stories that are still barely whispered and stories we think we understand and probably can’t even begin to grasp."
"We owe it to the memory of all those lost to the conflict to make progress now on addressing the past. Time is not going to lessen that debt. Rather it will compound it."
In a statement, President Michael D Higgins said: "Today, as people across the island of Ireland, and further afield, mark the 20th anniversary of the signing of the Good Friday Agreement, it is appropriate for us all to not merely recognise the achievement it was, the courage of its participants, but also we must commit to its endurance and deepening.
"The agreement heralded an end to decades of violence and the dawn of enhanced relations between the peoples of this island and with our close neighbours in Britain.
"As President of Ireland I pay tribute to all those people - politicians, activists, community workers and members of the Irish Diaspora alike - who have displayed unfaltering understanding, commitment and belief in the importance of developing and sustaining the peace process through word and action.
"On this day, as we remember and reflect on what has been achieved, we must also acknowledge our responsibility to build on the agreement and ensure the delivery of its promise of a better future based on partnership, equality and mutual respect."
Former British prime minister Tony Blair insists the agreement is still relevant today.
He observed: "For those people who say that the Good Friday Agreement doesn't matter, they should just remember what things were like.
"Before then we had decades... you remember, I remember what it was like waking up to tales of death and destruction, terrorism, sectarianism, hatred... people remember what Belfast was like at that moment, what it was like when you visited Northern Ireland.
"The Good Friday Agreement gave us the chance to put an end to all of that."
The North has been without a devolved government for over 14 months, since the collapse of the executive over the DUP’s handling of a botched renewable energy scheme.
The latest round of talks to restore power-sharing collapsed in February with the DUP and Sinn Féin at loggerheads over a range of issues – including the Irish Language Act.
Former US Special Envoy to Northern Ireland George Mitchell said the current crop of political leaders must to show leadership to end the current impasse and keep the peace:
"I don’t think they or anyone else should take for granted the absence of violence,” he said.
"I come from the US where we are plagued with the mass shootings; dozens, hundreds of our citizens including schoolchildren.
"No society can assume that they don’t have to do anything and things will be alright.
"It takes leadership – strong, effective, committed leadership."
Writing in The Telegraph, the British Secretary of State for Northern Ireland Karen Bradley insisted the restoration of power-sharing is the UK Government’s number one priority – even as the UK leaves the EU.
She said the UK government will stand firm behind an agreement that has "been the bedrock of all that has been achieved over the past 20 years."
"The restoration of devolved government at Stormont must be our number one priority," she said.
"I still believe that this is achievable and, as we reflect on the anniversary of the 1998 agreement, all of us need to redouble our efforts to make it a reality. Only then, with all parts of the agreement working properly, we will be in a position to build a Northern Ireland that is fit for the future. That is our goal and we are determined to achieve that."
When the agreement was signed 20 years ago, the dominant political parties in the North were the SDLP and the Ulster Unionists (UUP).
Mr Mitchell has warned the current crop of leaders that their "paramount objective must be to rekindle the spirit of the Good Friday Agreement and encourage resolution of political problems through democratic and peaceful means."
Additional reporting: Jack Quann