The expanded EU, which takes in a broad swath of the former Soviet bloc -- a region separated for decades from the West by barbed wire and Cold War ideology -- was widening to 450 million citizens at midnight to create a collective economic giant rivalling the United States.

A festive mood swept the newcomer nations, whose entry was hailed by EU leaders as "the end of the artificial divisions of the last century."

The EU's biggest expansion in its 47-year history brings in eight formerly communist countries -- the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Slovakia and Slovenia -- along with Cyprus and Malta.

Heads of state were gathering in Ireland, which is wrapping up its EU presidency, for a formal "Day of Welcomes" in Dublin today.

"For me, it's a great day," said Lenka Sladka, 24, a Prague university student. "Now we can freely travel or study everywhere. My parents could not even dream of it."

Enlargement stirred strong emotions both in the "new" Europe -- so dubbed by Defence Secretary Donald H. Rumsfeld before the U.S.-led war in Iraq, which most of the newcomers supported -- and in the "old."

The EU flag -- a circle of yellow stars on a blue field -- went up yesterday outside the presidential palace in tiny Slovakia, where parliament speaker Pavol Hrusovsky delivered a stirring reminder of how far the country has come since shaking off communism.

"In 1989, we cut up the barbed wire. Pieces of this wire have for us become a symbol of the end of the totalitarian regime," he said. "For the generation which lived in captivity of the barbed wire, the EU means a fulfilment of a dream."

French Prime Minister Jean-Pierre Raffarin said he gets emotional just thinking about it.

"I get tears in my eyes," he said while meeting with students from the 10 new countries. "I am part of a generation that believes in Europe. Europe is the force that prevents hate from being eternal. We must open our hearts to this new Europe."

Enlargement signals a "completely new chapter" in relations between Germany and Poland that were blackened by the Nazi occupation, German President Johannes Rau said in a landmark speech to the Polish parliament.

In the German town of Zittau, festivities were held in a grassy meadow on the Neisse River where Germany meets Poland and the Czech Republic. Makeshift pontoon bridges, adorned with the flags of the three countries and the EU, were set up to link the neighbors.

But the jubilation was tinged with frustration: fears in the newcomer nations of a loss of national identity and steep price increases, and worries in the EU's core 15 member states of a crush of immigrants as national borders gradually disappear.

Two bomb threats forced the closure of a key border crossing between the Czech Republic and Germany for more than four hours Friday. On Saturday, a group of avowed Czech "Euro-sceptics" planned a mock funeral to "bury" the country's sovereignty.

"Joining the EU is a necessary evil," said Zsolt Meszaros, 35, a Budapest doctor. "There are just too many uncertainties in all of this to make me more enthusiastic."

Chancellor Gerhard Schroeder sought to allay Germans' concerns that lower-paid workers from neighbouring Poland and other eastern countries would threaten their jobs. Greater trade across the enlarged Europe "will make us not poorer, but richer," he said in a nationally televised speech.

Musicians clad in EU national costumes played traditional songs on Prague's Wenceslas Square, where the mass demonstrations of former President Vaclav Havel's Velvet Revolution ended communism in the Soviet-dominated country he famously dubbed "Absurdistan."

In Lithuania, celebration organisers asked citizens to help the country literally outshine the other newcomer nations by turning on all their lights and building bonfires shortly before midnight. An American satellite was to photograph the region and beam the images back.

The mood was muted in Cyprus, which remains divided between ethnic Turks and Greeks. Cypriots in the Greek-controlled south decorated the main square in the capital, Nicosia, with EU flags, and musicians from around the world were performing into the night.

But no celebrations were planned in the Mediterranean island's Turkish-occupied north, where EU benefits and laws will not apply after Greek Cypriots rejected a U.N. reunification plan.

"We have worked very hard for this day so that once we are in a united Europe, Cyprus may also at long last cease to be divided," said Sotiris Georgiou, a refugee from northern Cyprus living in the capital.

In Hungary, an exuberant Prime Minister Peter Medgyessy watched two medieval bells being rung to symbolize unity and a return to the European mainstream.

Hungary "was always at the gates of Europe," he said.

"But the significant difference is that now we are inside the gates."