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 celebrations today is for European leaders to get together and show a brave face going forward."
She added: "They are signing a document today, they are taking beautiful pictures in a beautiful city -- all of these things to celebrate the 60th anniversary of the Treaty of Rome and to embark on a brave new world without the UK after Brexit
Protests as well as celebrations
But while the EU event was underway on Capitoline Hill, protests were going on close by. Demonstrators were on the streets in six separate protests in Rome, Nadeau said.
"There are anti-Europe protests; there are people from the far-right protesting; there are people from the far-left protesting -- people who want to see the European Union stay together, people who would like to see it break apart," she said.
In response to the demonstrations, 5,000 police officers were out on the streets and new closed-circuit TV cameras had been deployed to ensure public safety. Nadeau said authorities were concerned about the potential for clashes.
"So you've got two different things going on. You've got that beautiful ceremony inside the protected perimeter, and you've got what is probably going to end up being a very chaotic situation outside ... (with) people being able to say what they really think about the future of the European Union."
Marches across Europe
While no official crowd estimates have been released, CNN saw a large number of demonstrators at two pro-EU protests Saturday morning, where there were calls for a more federalist Europe without borders and a more welcoming attitude toward migrants.
But demonstrators were not just confined to the streets of Rome. Multiple marches were taking place across the EU as seas of people donned blue and gold and waved European flags in Warsaw, Poland, and Berlin.
In London, thousands marched toward Parliament Square, scene of Wednesday's deadly terror attack
, calling for Britain to remain in the EU.