The presidents of the EU's three main institutions have collected the Nobel Peace Prize in Norway's capital Oslo.
The EU was awarded the prize for its role in uniting the continent after two world wars.
At the ceremony there was applause when the leaders of France and Germany stood up, holding hands.
Critics say the award is inappropriate. They point out that the eurozone crisis has exposed deep divisions in the 27-nation bloc.
Most of Europe's national leaders were at the event, but not the UK's David Cameron.
The British prime minister's deputy, Nick Clegg - a longstanding advocate of the European project - represented the UK at the ceremony.
Nobel committee chairman Thorbjoern Jagland told the audience that in the current economic crisis "the political framework in which the union is rooted is more important than ever".
"We must stand together, we have collective responsibility," he said, warning of a risk of new nationalism in Europe.
The prize was received jointly by European Council President Herman Van Rompuy, European Commission President Jose Manuel Barroso and European Parliament President Martin Schulz. Mr Van Rompuy and Mr Barroso then gave a joint acceptance speech, in two parts.
Mr Van Rompuy paid tribute to the post-war leaders of France and Germany who had forged the EU by uniting their economic interests.
He praised "the EU's secret weapon - an unrivalled way of binding our interests so tightly that war becomes impossible".
"It is better to fight around the table than on a battlefield," he said, quoting Jean Monnet, one of the EU's founders.
German Chancellor Angela Merkel sat next to French President Francois Hollande at the ceremony in Oslo City Hall.
'Ich bin ein Europaer'
Mr Van Rompuy said the economic crisis was fuelling "the return of long-forgotten faultlines and stereotypes", but added: "Even such tensions don't take us back to the darkness of the past."
He ended by adapting the famous "Ich bin ein Berliner" quote from the late US President John F Kennedy during the Cold War.
He said he hoped future generations would "say with pride 'Ich bin ein Europaer', 'Je suis fier d'etre Europeen', 'I'm proud to be European'".
Four young Europeans, selected through an open EU competition, were in the delegation with equal status alongside the politicians.
The European Commission, which drafts EU laws, says the Nobel Prize money - about 930,000 euros (£755,000; $1.2m) - "will be allocated to children that are most in need".
The BBC's Europe correspondent Chris Morris says there has been a barrage of criticism - from Eurosceptics, peace activists and former winners of the prize.
Many of them question whether the EU should be given such an honour at a time when record unemployment and tough austerity policies, supported by European institutions, are causing serious social tensions in several member states.