The head of the European Commission says the bloc's 60th birthday declaration sets the scene for a growing mood of optimism.
Jean-Claude Juncker said after the leaders of 27 EU nations met that their Rome declaration is a good beginning for a wide-ranging discussion on the future of the bloc after Britain's departure. He added: "The atmosphere is now such that we can approach this with confidence."
Juncker said: "What we achieved in the days before Rome, and in the last few hours here in Rome, conveys something of an incipient optimistic mood — because, contrary to what was assumed, there was no clash, no big dispute between several conceivable paths."
The Rome declaration enshrines the principle of a multi-speed EU.
European Union leaders have signed the Rome declaration which has enshrined the principle of a multi-speed bloc, where some nations can move ahead while others stay on the sidelines on specific issues.
The declaration signed by 27 nations said that "we will act together, at different paces and intensity where necessary, while moving in the same direction."
The EU has often done that in practice in the past, with only 19 nations in the eurozone and not all members participating in the Schengen zone of borderless travel.
European Council President Donald Tusk has said on the 60th anniversary of the bloc's founding treaty that continued unity for the 27 nations remaining following the planned departure of Britain is the only way to ensure the bloc's survival.
Tusk told 27 EU leaders that "Europe as a political entity will either be united, or will not be at all" He spoke during a solemn session in Rome to mark the 1957 signing of the Treaty of Rome.
Tusk said that "only a united Europe can be a sovereign Europe in relation to the rest of the world. Only a sovereign Europe guarantees independence for its nations, guarantees freedom for its citizens."
Italy's premier says that European Union leaders must earn the support of their half-billion citizens and fend off rising nationalism on the continent by creating jobs and eliminating social inequality.
Paolo Gentiloni opened a summit in Rome marking the signing of the EU's founding treaty 60 years ago in precisely the same ornate hall on Rome's Capitoline Hill.
He observed: "We are a little more crowded in this hall," referring to the 27 EU members remaining after Britain plans to depart the union. Italy was one of the six founding members in 1957.
Gentiloni chastised the EU for being late on handling the migrant crisis and responding to demands to create jobs.
He said: "We must restore the trust of our citizens" through stimulating growth, reducing poverty and social inequality.
The European Union's trade commissioner says that she sees "a Europe that stands up for liberty, democracy and the rule of law," but adds that "the European project has many challenges and shortcomings."
Cecilia Malmstrom, a Swede, said Saturday on her blog that people need to discuss what they want and on the 60th anniversary of the EU, "we want to celebrate with dialogue instead of conflict, to create a future that we in good conscience can hand over to future generations."
Anders Fogh Rasmussen, a former NATO secretary-general and Danish prime minister, tweeted that "greater unity must come from more flexibility toward member states' visions."
European Commission President Jean-Claude Juncker says to have such a festive meeting like the 60th anniversary of the European Union founding treaty without British participation is "a very sad moment."
Juncker said that "Brexit, the exit of Britain, is a tragedy" for the 27 other nations meeting in Rome.
Britain voted last year to leave the bloc and is set to trigger the two years of divorce proceedings next Wednesday.
Germany's foreign minister says that his country must be careful not to be seen as lecturing smaller European Union countries, despite calls for a German leadership role.
Sigmar Gabriel wrote in an article for weekly Der Spiegel's online edition Saturday that "Europe is more than and above all often different from Germany." He added that smaller EU countries should view Germany, the bloc's most populous nation and its biggest economic power, as being interested in them rather than lecturing them.
Gabriel wrote: "We should counter temptations from Beijing, Moscow and Washington, which always want just to speak to us Germans, by noting that we are happy to play an important role and want to take responsibility — but that Europe is far bigger than Germany, and they can only have us together.
Residents of Rome are avoiding the city center as authorities brace for the possibility of violent protests during a European Union summit.
Some subway stops are closed, and buses have been rerouted away from the historic heart of the Italian capital hours before several planned marches.
Authorities fear anarchists might infiltrate anti-EU protests set for the afternoon.
Leaders from 27 EU nations gathered on the ancient Capitoline Hill on the 60th anniversary of the founding treaty of the EU, whose unity is now being sorely tested.
One march is organized by far-right opponents to the EU, while another is organized by far-left opponents.
Also scheduled is a pro-EU march, which could draw hundreds of Britons who live in EU countries and fear complications from Britain's exit from the union.
European Union leaders are gathering in Rome to mark the 60th anniversary of their founding treaty and chart a way ahead following the decision of Britain to leave the 28-nation bloc.
On a day of ceremonies Saturday, the 27 leaders are set to approve a Rome declaration to commit to a united future and see how to deal with the myriad crises which has beset them over the past decade.
It looks like the blueprint will be adopted without any problems after both Poland and Greece lifted their objections on the eve of the summit.
Britain says that it will trigger the negotiations to leave the bloc on March 29, only days after the summit.