European Union leaders celebrated half a century of unity Sunday by hailing the bloc's achievements in bolstering peace, democracy and prosperity, then pledged to end two years of deadlock over plans to radically overhaul the way the EU does business.

"Stagnation means regression," German Chancellor Angela Merkel warned her fellow leaders, urging them to get going on discussing a new EU treaty. "We must always renew the political shape of Europe in keeping with the times."

Merkel, whose country has the rotating EU presidency, appealed for support for a German "road map" that calls on the EU to adopt a new treaty by 2009. That agreement would replace the draft constitution rejected by French and Dutch voters in referendums in 2005, plunging the EU into profound crisis.

She expressed hope that the EU can agree in June, before her presidency ends, to set up an intergovernmental conference that would come up with a new proposal.

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"We are very much agreed that the EU, as it is, is not sufficiently capable of acting," she said at a concluding news conference.

However EU leaders are divided over how much of the old constitution should be retained, and those disagreement sprang up again even amid celebrations of the EU's 50th birthday, accompanied by the upbeat strains of Beethoven's Ninth Symphony, in reunited Berlin a few blocks from where the Berlin Wall once stood.

Germany and Italy want to preserve much of the substance of the original text — Italian Prime Minister Romano Prodi told the summit it was "a very solid basis" on which to build.

However, the Dutch government this week insisted a new treaty "must, in content, scope and name, convincingly differ" from the constitution. The Poles, Czechs and British are even more wary about any changes that shift power from national capitals to EU headquarters.

Finding a solution won't be easy. Prodi said the birthday celebrations marked the end of "a period of mourning" over the failed constitution, but acknowledged the EU needed to "rediscover something of its creative madness" to find a way forward.

Substantive talks on the constitution are unlikely to get under way until after the French presidential election in April, which will see the departure of Jacques Chirac, who was attending his last EU summit in Berlin after almost 12 years in office.

Reflecting the divisions, the Berlin Declaration — marking 50 years since the signing of the Treaty of Rome that led to the creation of the EU — did not include the word "constitution." Instead, the 27 presidents and prime ministers gave a commitment only to place the EU on a "renewed common basis" by the time bloc's 490 million citizens elect a new European Parliament mid-2009.

Agreement on even that vague wording needed some last-minute diplomatic arm-twisting from Merkel — whose government holds the EU's rotating presidency. Czech President Vaclav Klaus — a frequent critic of the EU — was angered by the whole process, accusing the Germans of failing to hold "a democratic discussion" with their partners.

Despite the criticism, EU leaders are aware of the need to overhaul the EU, which has grown from six original members in 1957 to 27 today, expanding its original role promoting economic cooperation into an integrated bloc with a shared currency, common borders and cooperation on areas ranging from the environment and immigration to defense and foreign policy.

Most feel that the EU's internal workings have failed to keep pace with the changes, leading to policy gridlock and handicapping Europe's efforts to develop a more assertive global role. The stalled constitution would have created a bill of rights, a full-time EU president and foreign minister and faster decision-making.

Merkel hoped the weekend celebrations in Berlin would rekindle some euro-enthusiasm. The Germans organized a mix of European cultural festivities ranging from Beethoven at the Berlin Philharmonic to Scottish folk songs, flamenco dancing and Danish hip-hop beneath the Brandenburg gate.

Merkel — who grew up in communist East Germany — underscored the significance of the location of the ceremonies.

"We are celebrating in Berlin, a city that until 18 years ago was divided by a Wall, by barbed wire, by soldiers with orders to shoot, in which people paid with their lives for seeking to escape to freedom," she told the summit.

From the other end of Europe, European Commission President Jose Manuel Barroso grew up under Portugal's rightist dictatorship. He recalled that over half the countries were under dictatorship or foreign rule 50 years ago.

Merkel and Barroso both turned on Belarus, a neighbor which EU leaders have denounced as Europe's last dictatorship. As police and pro-democracy protesters faced off in the streets of Minsk on what the opposition calls "freedom day," Barroso assured the people of Belarus they had a place in the "European family" and promised EU aid if the government adopted reforms to end its "self-imposed isolation."

The EU also raised the "unbearable" suffering in Darfur and raised the possibility of stronger sanctions against Sudan if it fails to uphold U.N. resolutions aimed at stemming fighting in the region.

"Even today, our thoughts are with the people in ... Darfur. The suffering there is unbearable," Merkel said, after Hollywood star George Clooney joined a high-powered panel of European intellectuals in urging stronger European action.

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