Good Friday Agreement: Cameron hails 15th anniversary
The prime minister has hailed the Good Friday Agreement as a "truly momentous event" in Northern Ireland's history, on the 15th anniversary of the accord.
The deal, also known as the Belfast Agreement, was signed on 10 April 1998.
It followed years of intensive peace talks between political parties in Northern Ireland and the British and Irish governments.
David Cameron said that over the past 15 years, Northern Ireland had "come a long way" but much remained to be done.
Tony Blair, who was prime minister at the time of the agreement, said what had been achieved was historic.
"I think the abiding memory for me is being locked up in Castle Buildings for several days as the thing lurched from crisis to crisis and yet emerging at the end of it absolutely exhausted after barely any sleep over those days with an historic agreement," he said.
Mr Blair said he could understand some subsequent criticisms of the government from both the Ulster Unionist Party and the SDLP.
"The risk with all these situations - and it's a very hard risk to avoid - is that, because those at the furthest end of the political spectrum had to be on board as well, if you're not careful you spend a lot of time trying to bring them on board," he said.
"The people who are on board already, who have taken massive political risks to be so, obviously not only feel discomforted, but rejected."
The SDLP said the Good Friday Agreement was a "seminal point in modern Irish history" and called for a public holiday to mark the anniversary.
Its leader, Alasdair McDonnell, said it was a "unique exercise in democracy which mapped out the end to that conflict and laid the foundation stone on which the new Ireland for the 21st century will be built".
Sinn Fein leader Gerry Adams said: "The power-sharing structures have ensured the checks and balances necessary to prevent the political abuses and discrimination of the past.
"The agreement has secured remarkable progress in the areas of policing and justice, demilitarisation and arms, discrimination and sectarianism, equality and human rights, and the Irish language."
The 65-page document paved the way for the establishment of a devolved government at Stormont and a copy was sent to every house in Northern Ireland.
Six weeks later, it was ratified by referendums on both sides of the Irish border, with 71% of people in Northern Ireland and 94% in the Republic of Ireland voting in favour of the deal.
In a statement on Wednesday, Mr Cameron said: "After decades of division and terrorism, the agreement heralded a new beginning for relationships within Northern Ireland, between Northern Ireland and the Republic of Ireland, and across these islands.
"At this distance it is easy to forget just how painstaking and lengthy the process was that eventually led to the agreement."
Secretary of State Theresa Villiers said: "Northern Ireland has come a very long way since the Belfast Agreement was reached 15 years ago. Few can deny that life here has changed for the better.
"But at too many levels society remains deeply divided and huge challenges remain if the hopes enshrined in the agreement are to be properly fulfilled.
"We urgently need to tackle sectarianism and segregation in order to build a more cohesive society."
Many unionists, particularly the DUP, were angered by the proposals and the party's leader then, the Reverend Ian Paisley, led a vigorous referendum campaign for a "no" vote.
The "no" campaign argued that plans for mandatory coalition, enforced power-sharing with nationalists, was anti-democratic.
They also said that proposals to increase cross-border co-operation with the Irish government would undermine British sovereignty.
Under the terms of the deal, paramilitary prisoners were granted early release, which caused great distress to many victims of the Troubles and their families.
The prime minister said in his statement that although the agreement was not perfect, it was "a massive step forward from what had gone before, a clear manifestation that politics and democracy would triumph over violence".
'Winners and losers'
He said the architects of the agreement, and those who "displayed remarkable political courage in pushing it forward", deserved thanks and should "not be shy" about trumpeting its achievements.
"There is still a strong tendency in Northern Ireland to view politics as a zero sum game, in which there are only winners and losers," Mr Cameron said.
"That is not the case with the Belfast Agreement. I firmly believe that all parts of the community were winners on 10 April 1998."
The prime minister said Northern Ireland's best days still lay ahead and described the accord as "the platform to build a new, confident, inclusive and modern" society.
The SDLP leader said agreement still provided the "best hope, perhaps the only hope, of creating and shaping a reconciled and prosperous society".
Mr McDonnell said: "The Good Friday Agreement transformed relations between our peoples and our nations. It truly is the people's agreement, endorsed overwhelmingly in the only true exercise in national self-determination since 1918.
"It is a seminal point in modern Irish history, which defines our values, ambitions, institutions, our political system, and the potential to build a new society."
Labour's shadow secretary of state for Northern Ireland, Vernon Coaker, said the 15th anniversary was an opportunity "to reflect on the progress made since the signing of the Good Friday Agreement, and to renew our commitment to working for peace and prosperity".
"The peace process in Northern Ireland was one of Labour's proudest achievements in government," he said.
"Many of my colleagues today will be thinking of Mo Mowlam and the huge contribution she made.
"The Good Friday Agreement shows it is only by working together that we get things done. That is as true today, for the British and Irish governments and the Northern Ireland parties, as it was in 1998."