The informal summit in Rome sees leaders from across Europe renew their commitment to the EU in light of Britain's impending exit.
British Prime Minister Theresa May -- who is not attendance -- is expected to trigger Article 50
and notify the EU of the UK's intention to leave the bloc on Wednesday.
On Saturday, the remaining member states adopted the Rome Declaration, which calls for enhanced cooperation and unity.
It reads: "We, the Leaders of 27 Member States and of EU institutions, take pride in the achievements of the European Union: the construction of European unity is a bold, far-sighted (endeavor). Sixty years ago, recovering from the tragedy of two world wars, we decided to bond together and rebuild our continent from its ashes.
"We have built a unique Union with common institutions and strong values, a community of peace, freedom, democracy, human rights and the rule of law, a major economic power with unparalleled levels of social protection and welfare."
'A brave face going forward'
The Treaty of Rome was signed on March 25, 1957, creating the European Economic Community of France, Germany, Italy and the Benelux (Belgium, the Netherlands and Luxembourg).
European Commission President Jean-Claude Juncker spoke with optimism, urging member states not to be sidetracked by disgruntled voters and offering to listen to concerns while maintaining solidarity.
"Today we renew our vows and reaffirm our commitment to an undivided and indivisible union," Juncker said, according to Reuters.
European Council chief Donald Tusk hammered the point home, asking: "Why should we lose our trust in the purpose of unity today? Is it only because it has become our reality? Or because we have become bored or tired of it?"
Speaking to reporters at the summit, German Chancellor Angela Merkel called for better border protection, saying: "We are looking toward the future. We want a safer Europe and a protective Europe, which means we need to protect our external borders better."
CNN's Barbie Nadeau said that "the whole purpose of these