EU leaders have adopted a declaration calling for some of the reforms proposed in the bloc's ill-fated constitution to be carried out by 2009.
Berliners were treated to fireworks at the Brandenburg Gate
The "Berlin Declaration" was issued to mark 50 years of the union, which was founded by the 1957 Treaty of Rome.
It says the EU should be placed on a "renewed common basis" before the 2009 elections to the European Parliament.
German Chancellor Angela Merkel said a conference to agree a new treaty could be held later this year.
The ceremonies came after a night of revelry in the German capital, with a gala concert and a lavish dinner.
Street parties were taking place in Berlin on the Sunday while capital cities across the EU have been celebrating since Friday with rock concerts, raves and special late-night museum openings.
A "European Village" opened in the heart of Rome's historic centre, which has been shut off to traffic for the occasion.
The Berlin Declaration looks back to a time before the EU, when Europe, it says, was "an idea, holding out hope of peace and understanding".
It contrasts the previous eras of war and division with the peaceful times since the formation of the union.
The unnatural division of Europe is now consigned to the past
We, the citizens of the European Union, have united for the better
We preserve in the European Union the identities and diverse traditions of its member states
We are united in our aim of placing the EU on a renewed common basis before the European Parliament elections in 2009
"We, the citizens of Europe, have united for the better," the declaration reads.
There is no explicit mention of the most divisive issues - future enlargement to admit Turkey and the Balkan nations, and the EU constitution.
But correspondents say the call to place the EU "on a renewed common basis" before the June 2009 elections to the European Parliament is a coded reference to institutional reforms.
Ms Merkel said the Portugal "could organise" an intergovernmental conference when it takes over the presidency from Germany in July.
She was echoing comments by Luxembourg's Prime Minister, Jean-Claude Juncker, who told German radio that the "ideal timetable" would see the draft of a new treaty agreed during the Portuguese EU presidency.
British Prime Minister Tony Blair also said the EU needed "more effective rules", adding that the "sooner it is resolved the better".
EU leaders have been attending a series of celebratory events
However, Polish President Lech Kaczynski said that ratifying a new treaty by June 2009 was "unachievable".
He said that he and some other EU leaders understood the wording of the Berlin Declaration to mean that the treaty could be agreed by 2009, but not ratified.
European leaders are divided about how closely any new treaty should resemble the constitution.
Germany and Italy are keen to preserve it largely intact, while the UK and the Netherlands want a very different text.
Some leaders also believe the project will come unstuck again if member states decide to ratify the treaty by referendum.
But British EU commissioner Peter Mandelson told the Sunday AM programme on BBC 1 that big changes to the EU had been carried out in the past without a public vote.
"I suspect that the changes that will be proposed and I hope they will be will be rather fewer with fewer constitutional implications then the single market that was created in the eighties and didn't have a referendum...
"The Maastricht treaty that didn't have a referendum provided by the then Conservative government...
"So I think it depends on what is being proposed as to whether we think it crosses the threshold for a referendum," he said.
But a poll by a British Eurosceptic think tank, Open Europe, suggests that three-quarters of Europeans would like a referendum on any new treaty giving more power to the EU.
According to the poll, carried out in all 27 EU countries, 41% of people in the EU would Yes in such a referendum and the same proportion, 41%, would vote against.
However, a majority would vote No in 16 EU countries, including Germany.