The European Union has officially welcomed its 10 newest members at a simple flag-raising ceremony in Dublin. The EU's biggest ever expansion includes former communist nations that were cut off from the European mainstream by the Cold War.

Leaders from all 25 EU countries gathered on the lawn outside the residence of Ireland's president to celebrate what many central and eastern Europeans regard as the return of their nations to the European family.

Polish President Alexander Kwasniewski, who leads the biggest of the new members, was asked by reporters how he feels after years of arduous negotiations to meet EU membership requirements.

"I'm happy because, really, this is a great day for us, for Poland, for Europe, and I think finally we will realize our dreams," said Aleksander Kwasniewski.

For Poland, the Czech Republic, Slovakia, Hungary, Slovenia, Estonia, Latvia and Lithuania, joining the EU crowns more than a decade of often painful economic reforms that followed the collapse of communist rule.

Also entering the bloc Saturday were the Mediterranean island republics of Cyprus and Malta.

The formal ceremony in Dublin came after hundreds of thousands of revelers in the new member states celebrated the final closing of Europe's east-west divide.

Ireland's president, Mary McAleese, set the tone for a day in which the EU took a break from its routine disputes over money, power and fishing quotas.

"It's a momentous day of celebration, when the past is laid to rest and the future is anticipated with great hope," she said.

But as EU leaders hail their union's latest expansion as a historic triumph, they are also worried that it will be costly and might paralyze decision-making.

The move eastward and southward increases the bloc's population by 75 million, its territory by 25 percent but its gross domestic product by barely five percent.

The challenges the EU faces are to integrate these poorer countries, stay manageable with 25 states around the table and control immigration and organized crime as borders move east to adjoin former Soviet lands such as Russia, Ukraine and Belarus.