Barack Obama has delivered an earnest speech on peace in Northern Ireland, to an invited audience of mainly schoolchildren.
In the region ahead of the G8 summit, the President told the country's politicians to keep working towards a permanent peace.
He said that, fifteen years after the Good Friday peace agreement was signed the world was watching for the next stage of the process, and said that the region can be a model for building peace in other conflicts. He also hinted it was time for an end to segregated education and housing.
He said: "You need to get this right. You set the example for those who are seeking peace to end conflicts of their own.
"You are their blueprint to follow. You are the proof of what is possible. Hope is contagious. They are watching to see what you do next."
During the address, President Obama promised that America would continue to support the Stormont political system.
He added: "Peace is not just about politics. It is about attitudes, a sense of empathy and breaking down barriers in hearts."
Peter Robinson and Martin McGuinness, the First and Deputy First Ministers of the Stormont power-sharing executive, greeted the president on arrival at Waterfront Hall and held a brief private meeting before Mr Obama appeared on stage.
Mr Obama acknowledged the challenges that exist.
"There are still people who have not reaped the rewards of peace, there are those who are not convinced that the effort is worth it," he said.
"There are still wounds that have not been healed and communities where tension and mistrust hangs in the air. There are walls that still stand, there are still many miles to go."
He said it was within his audience's power to change that.
"Whether you are a good neighbour to someone from the other side of past battles, that is up to you," he warned.
"Whether you treat them with the dignity and respect they deserve, that is up to you.
"Whether you let your kids play with kids who attended a different church...that is up to you."
The president made a direct appeal for the young people in the audience to do all they could to ensure that Northern Ireland's peace was lasting, saying: "Peace is indeed harder than war. It's constant fragility is part of its beauty. A bullet need only happen once but for peace to work we need to be reminded of its existence again and again and again.
"You must remind us of the existence of peace. Remind us of hope again, again and again that despite resistance, setbacks and despite hardship, despite tragedy... You need to remind us of the future.
"For those who choose the path of peace, the United States of America will be with you every step of the way. We will always be a wind at your back. This little island inspires the best of things. For this little island the best of days are yet ahead."
Air Force One touched down at Aldergrove International Airport, 20 miles north of the city, at 8.35am amid the biggest security operation ever mounted in Northern Ireland.
Mr Obama was flown by Marine One helicopter to George Best Belfast City Airport and then travelled by road to the convention centre a mile away.
Hundreds of police 4x4s lined the streets, while the Police Service of Northern Ireland helicopter patrolled the skies over the city and specialist boat units kept watch from the River Lagan.
Thousands of extra police officers were deployed to the province ahead of the G8 summit, including 3,600 from other forces in England and Wales.
Even though Mr Obama visited the Republic two years ago, this is his first trip to Northern Ireland.
Later he will travel to the luxury Lough Erne golf resort in Enniskillen, Co Fermanagh, where he will join leaders including Russian Premier Vladimir Putin and German Chancellor Angela Merkel for the two-day G8 economic conference.
Video: UK President Barack Obama delivers speech