The once-communist states of the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Slovakia and Slovenia officially joined the EU family this weekend, along with the Mediterranean islands of Cyprus and Malta.

The expansion from 15 to 25 rounded out the world's biggest trading bloc, which now stretches from Ireland's Atlantic coast to the Russian frontier and is home to 455 million people.

It is by any measure a formidable entity. And the enlargement is all the more impressive coming just 15 years after the Berlin Wall collapsed.

The flags of all 25 nations of the new-look EU were raised in the presence of the countries' leaders at a solemn ceremony Saturday in Dublin's vast Phoenix Park.

"We welcome them with pride. We welcome them with hope. And we invite all the people of Europe to celebrate with us," Irish Prime Minister Bertie Ahern said in a speech.

"We pause to reflect on what we in the European Union have created," he said. "We must never forget that from war, we have created peace. From hatred, we have created respect. From division, we have created union."

The leaders headed on to dinner in the Farmleigh state guesthouse, toasting the enlargement over champagne as well as wines from France and EU entrant Slovenia.

From Ireland's Atlantic coast to the Russian frontier, from the fringes of the Arctic to the sun-kissed Mediterranean, enlargement was celebrated with fireworks, street concerts and ringing declarations of a continent reunified.

But on the fringes of the 1,762-acre (712-hectare) Phoenix Park, Europe's largest enclosed space, trouble was brewing at the climax to a series of peaceful May Day protests in the Irish capital.

For the first time in Irish history, the Garda national police turned water cannon on leaders of a march by about 700 protestors late Saturday trying to gain access to the park.

Twenty-nine people were charged with public order offences and one policewoman was treated in hospital after she was hit on the head by a flying bottle, police said on Sunday.

"I am disappointed that the disturbances happened," Ahern said, while praising police for their handling of the situation.

The clashes marred Ireland's welcome to the 10 new EU members on a day that, in the words of Irish Nobel laureate poet Seamus Heaney, "hope and history rhyme".

Between champagne toasts, however, the EU leaders could reflect on the many challenges still facing the EU -- including the economic gulf between east and west, and the vexed issue of a constitution to keep the EU on its rails.