Northern Ireland has a new power-sharing government in an historic day at Stormont.
DUP leader Ian Paisley and Sinn Fein's Martin McGuinness took office as first and deputy first ministers as five years of direct rule ended.
Tony Blair and Bertie Ahern witnessed the creation of the new executive.
Mr Blair said that the day's events offered the chance for Northern Ireland to "escape the heavy chains of history" and "make history anew".
In October 2002, allegations of intelligence gathering within Stormont led to the suspension of power-sharing institutions. A subsequent court case collapsed.
"Look back and we see centuries pock-marked by conflict, hardship, even hatred, among the people of these islands," the prime minister said.
Irish Premier Bertie Ahern thanked all the politicians who have been involved in the peace process, but reserved special praise for Mr Blair.
Mr Paisley said: "Today we are starting upon the road which I believe will take us to lasting peace in our province."
He added: "I welcome the pledge we have all taken to that effect today... that is the rock foundation upon which we must build."
Deputy First Minister Martin McGuinness said he wished Ian Paisley all the best as they began "the greatest, yet most exciting, challenge of our lives".
"We must overcome the difficulties which we face in order to achieve our goals and seize the opportunities that now exist," he said.
Mr McGuinness said he was confident he and the DUP leader could work together.
Both Mr McGuinness and Mr Paisley paid tribute to DUP assembly member George Dawson, who died on Monday evening.
Ulster Unionist leader Sir Reg Empey said he was delighted that the people of Northern Ireland now had a government of their own.
SDLP leader Mark Durkan said: "What today shows is that when finally you have a government setting a deadline and setting terms and keeping to them, you can get somewhere."
William Hay was appointed as the new speaker, replacing the outgoing Eileen Bell.
In nominating the DUP assembly member, Mr Paisley said the speaker in the next assembly would be from the nationalist community.
Protesters tried to block the PM's motorcade
Demonstrators protesting against the war in Iraq were forcibly removed by police after they attempted to block the arrival of Mr Blair's motorcade.
The protesters, who had been standing in front of Parliament Buildings, ran down the hill to the Carson Statue and lay down on the road.
Ministers from the four main parties took the pledge of office, which includes support for the police.
The return of devolved government follows an historic meeting in March between Mr Paisley and Sinn Fein President Gerry Adams, where they agreed to share power.
Northern Ireland Secretary Peter Hain said he was confident the parties would make a go of it.
VIP guests at Stormont included US Senator Ted Kennedy, the DUP leader's wife Baroness Paisley and Peggy McGuinness, the deputy first minister's mother.
Also attending was Jeanette Ervine, the widow of Progressive Unionist Party leader David Ervine, who died in January.
The first meeting of the new power-sharing executive is scheduled for later this week.