The leaders who brokered a peace deal for Northern Ireland in 1998 marked its 20th anniversary on Tuesday by warning that a hardening political divide and Britain's exit from the European Union were creating new dangers for the region.
Former US president Bill Clinton and ex-prime minister Tony Blair were also among those invited to attend the event taking place at Queen's University Belfast.
Despite this, the future of Stormont is again hanging in the balance after it collapsed over a year ago.
A challenging and complex negotiation period saw the end of a 30 year conflict that claimed over 3,000 lives.
The accord included the early release of about 500 paramilitary prisoners within two years even though IRA arms had not yet been destroyed. And there are hard questions over how to avoid a hard border with the Republic after Brexit.
However, he maintained that the Good Friday Agreement was robust enough to weather the challenges of Brexit and that a solution would be easy.
"The Tory government has actively encouraged the most negative, intransigent and sectarian elements of political unionism to attack and undermine the Good Friday Agreement", Gerry Adams, the former Sinn Fein leader who also helped negotiate the agreement, said in a speech on Tuesday.
"We have to be very, very careful", said former U.S. Senator George Mitchell, who chaired the talks that led to the agreement, when asked by Irish state broadcaster RTE if there was a danger of a return to violence.
Senator Mitchell added: "The current problems in Northern Ireland are hard, they are serious and they must be resolved but at the same time we should not hold Northern Ireland to a higher standard to everywhere else in the world".
Paramilitaries undertook to destroy weapons and the IRA did so in 2005.
The agreement won the overwhelming support of a majority following referendums in both parts of the island and served as a blueprint for how to solve conflict around the world.
"What we must do now is not to despair, not to look backwards, but to reaffirm to the people and the leaders of Northern Ireland - our dedication to the principle of political differences here and elsewhere must be resolved through peaceful and democratic means, not through the use or threat of violence".
"Do not underestimate the value of the fact that this was done by people from different parties, of different persuasions and not one damn time in the whole process did anyone ever suggest that democracy itself should be abandoned; that the results of elections should not be respected, that some people's votes should be denied, discredited or that facts were not facts".