EU President Donald Tusk called for leadership to steer Europe out of crisis at a special summit in Rome to mark the 60th anniversary of the bloc's founding treaties.
"Prove today that you are the leaders of Europe, that you can care for this great legacy we inherited from the heroes of European integration 60 years ago," the former Polish prime minister said in a speech.
The 27 leaders marked the 60th anniversary of the bloc's founding by renewing their commitment to a common future.
Against a backdrop of crises and in the absence of the departing Britain, the leaders signed a new Rome declaration, six decades after the six founding members signed the Treaty of Rome and gave birth to the European Economic Community.
Taoiseach signs formal declaration of unity at 60th anniversary of founding of EU pic.twitter.com/qVqqPhLqxh— RTÉ News (@rtenews) March 25, 2017
Conspicuous by her absence was British Prime Minister Theresa May, who will write Mr Tusk on Wednesday ;to formally announce that its second-biggest economy will leave the Union in negotiations over the coming two years.
Britain shunned the new European community at its creation, but finally joined in 1973. Its people voted to quit last June.
Without the so-called Brexit, it might have been a modestly hopeful summit in the palazzo where old foes France and Germany, with Italy, the Netherlands, Belgium and Luxembourg, signed the original treaty.
All the bloc's economies are growing after a slump that has blighted the past decade and recent border chaos has largely abated as refugees are, for now, being held in check.
But Brexit has undermined the self-confidence of a Union that has helped bring peace and growing prosperity to the continent, and has encouraged eurosceptic nationalists challenging governments from Stockholm to Sicily.
It has also amplified the petty frictions among the more than two dozen national governments and obliged leaders' aides to water down a grand birthday declaration of unity.
After days of carping from Poland and Greece, seeking to show home voters they were getting Brussels to give assurances about equal treatment and social welfare, the Rome Declaration the 27 will sign offers ringing phrases about peace and unity.
"We have united for the better," the text concludes. "Europe is our common future."
But it may disappoint those who think more ambition and coordination is the answer to malaise.
At the Vatican yesterday, Pope Francis told them that their Union had achieved much in 60 years but that Europe faced a "vacuum of values".
He condemned anti-immigrant populism and extremism that he said posed a mortal threat to the bloc.
"When a body loses its sense of direction and is no longer able to look ahead, it experiences a regression and, in the long run, risks dying," said the pontiff.
Their response, he said, should be to promote Europe's ideals and values with more vigour and passion.
He urged states to show more "solidarity".
And the first non-European pope in over 1,000 years reminded them of the diminishing share of the world's wealth and people in Europe.
They were a "peninsula of Asia", Francis told them, urging them to remain open to the rest of the world.
Meanwhile, thousands of people marched through London to protest against Britain leaving the European Union.
The Unite for Europe march was due to end with a rally in Parliament Square, scene of this week's attack in which four people died.
Marchers observed a minute's silence in memory of the victims at the start of the demonstration.
In bright sunshine, they waved EU flags and banners with slogans like "So what's the Plan" and "Stop Brexit" as they made their way to parliament.
Draft EU-Brexit guidelines 'should reflect' Ireland's position
Taoiseach Enda Kenny has said that the draft EU negotiating guidelines, due to be released within 48 hours of Mrs May triggering Article 50, should reflect the "unique and particular set of circumstances" facing Ireland once Brexit becomes a reality.
These included the Good Friday Agreement, the Common Travel Area, and the nature of the Irish land border.
"We have enjoyed acquired rights since 1922 of travel to and from Britain and Northern Ireland and to Ireland, and also in respect of residence, the right to work, and social welfare opportunities.
"We can't negotiate with Britain until Article 50 is triggered. We've set out priorities are, and we will sit on the side of the EU27 when the negotiations start," Mr Kenny said.
The Taoiseach also said he had spoken to Mrs May by phone about the terror attacks in London and had offered the Government's sympathy for the bereaved families and support for the British government in its fight against terrorism.
"The response next Friday [to the triggering of A50] will be a draft response, hopefully for a decision [by EU heads of government] on the 29th of April."