The meeting was also a chance for them to counter rising challenges after Brexit and the perceived threat of populist parties.
The only EU leader missing from the showpiece event was UK Prime Minister Theresa May who, on Wednesday, will trigger article 50 to start Brexit talks.
In a declaration issued to mark the 60th anniversary of the EU founding treaty, the leaders from 27 member states said, "Europe is our common future" and voiced optimism that they can hold the bloc together following loss of Britain.
Several EU leaders spoke of their hopes and fears, including European Commission President Jean-Claude Juncker who said, "Only by staying united can we pass on to future generations a more prosperous, a more social and a safer Europe. A Union of solidarity, that is strong, that is generous both at home and in the wider world. A Europe that takes up the major challenges of the day and that does not lose itself in the detail."
Juncker added, "But let us not lose perspective either. However daunting our challenges may feel today, they are in no way comparable to those faced by our founding fathers."
European Council President Donald Tusk said, "I was eight years old when the Community established a single council and a single commission through the Merger treaty; the road I then took to school every day still led through the ruins of the burnt city. For me, the Second World War is not an abstraction."
He added, "I lived behind the Iron Curtain for more than half of my life, where it was forbidden to even dream about those values. Yes, back then, that really was a two-speed Europe. And that is why today I have the right to loudly repeat this simple truth: that nothing in our life is granted forever - that to build a free world requires time, great effort and sacrifice."
Parliament President Antonio Tajani , who led a delegation of Vice-Presidents, Quaestors and political group leaders, said, "The European institutions cannot remain aloof in their ivory towers. We parliamentarians, first and foremost, and everyone who holds a position of responsibility, have a duty to open the door, go out into the world, to listen and discuss."
Committee of the Regions President Markku Markkula agreed, saying, "We must be proud of Europe's achievements over 60 years of peace and prosperity. My message here in Rome is clear: our Union will only survive and deliver on its promises if citizens and their regional and local communities are personally engaged. The European Committee of the Regions stands ready to make it happen."