Jean-Claude Juncker, the president of the European Commission, has said it was a "tragedy" that the EU was celebrating its 60th anniversary on Saturday without Great Britain.
"Brexit, the exit of Britain, is a tragedy [for the 27 other member states," Mr Juncker said.
EU leaders have put on a brave face for the summit, on top of Rome’s Capitoline Hill,where they will renew the EU’s marriage vows in a ceremony to mark 60 years since the signing of their founding document, the 1957 Treaty of Rome.
Looming over the summit will be Theresa May’s decision to trigger Article 50 next week and begin formal talks to secede from the Union – a reality reflected in the fact that Mrs May will be absent from Saturday’s line-up of leaders.
Juncker: #EU60 "We are not proud enough of what we have achieved in Europe".— Peter Foster (@pmdfoster) March 25, 2017
Preparations for celebration have been marred by deep divisions among EU members, with Poland and Greece both threatening to refuse to sign a formal declaration unless given concessions on issues, including immigration and austerity.
The ceremony began on a solemn note, with the leaders of the 27 members states stepping up to sign the Rome Declaration, a process which drew attention to Britain's absence at the event.
"Today we renew our vows and reaffirm our commitment to an undivided and indivisible Union," Mr Juncker told them.
Donald Tusk, the president of the European Council, was also at the ceremony, where he insisted that a united Europe was not the same as a "bureaucratic model."
"Europe as a political entity will either be united, or will not be at all...it is a set of common values and democratic standards."
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.
Additional reporting - AP