At the stroke of midnight (10 p.m. GMT), eight former communist countries -- the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Slovakia, and Slovenia -- along with the Mediterranean islands of Cyprus and Malta, became official EU members.
In the single largest expansion in the EU's 47-year history, the union has grown from 15 to 25 members. It now has some 450 million people, making it the world's largest free-trade area.
Later today, government leaders of all 25 member states are expected to attend an official ceremony in Dublin, the capital of Ireland, which currently holds the rotating EU Presidency.
Last night, there were celebrations and fireworks across Europe.
European Commission President Romano Prodi paid tribute to the people of both the new and old EU states. "It took courage, determination, and a lot of effort from the peoples and political forces in the new member states to get this far," Prodi said. "It took vision and generosity from the peoples and the leaders in the current European Union. Five decades after our great project of European integration began, the divisions of the Cold War are gone, once and for all. And we live in a united Europe."
Crowds were estimated at 100,000 in the Hungarian capital Budapest, 40,000 in Warsaw, and tens of thousands in the Czech and Lithuanian capitals, among others.
"Five decades after our great project of European integration began, the divisions of the Cold War are gone, once and for all. And we live in a united Europe." -- European Commission President Romano Prodi
Lech Walesa -- whose Solidarity movement toppled communism in Poland in 1989 -- said his country's entry into the EU fulfilled his "dreams and lifetime work."
"We are returning to where we belong, to a community that shares the same values and visions," Estonian Prime Minister Juhan Parts said.
"Hungary has returned to Europe," Hungarian Prime Minister Peter Medgyessy announced triumphantly. "It has deserved it; 10 million people have worked for it." Medgyessy hit a gong marking midnight and the start of the new era.
"Ladies and Gentlemen, we are making history," Polish President Aleksander Kwasniewski enthused at a midnight ceremony to hoist the EU flag in the capital Warsaw.
But Kwasniewski was also heckled by a group of Polish Euro-skeptics, Reuters noted. Self-styled "Euro-realist" and Czech President Vaclav Klaus meanwhile delivered a sobering television speech.
"We are not joining Europe, because we have been there for a long time, we have always been there, even at the times of our greatest subjugation," Klaus said. "We are joining the European Union and our task is thus much more prosaic: we must learn how to live in the Brussels structures and the complicated supranational entity those structures have created."
Belgian Prime Minister Guy Verheugen called the expansion of the EU "a new era for Europe."
In France, President Jacques Chirac hailed enlargement as a giant leap forward for Europe.
The first celebrations were in Cyprus and the Baltic republics,
since they are one time zone east of the others.
Political leaders and ordinary people hailed the final closing of Europe's east-west divide, 15 years after the Berlin Wall fell and 60 years after the end of World War II.
But even as they marked this historic enlargement, several leaders said it should not be the bloc's last.
"Today Poland is returning to its European family. But the door to the European Union must be left open," Polish President Kwasniewski said. "All those whose ambitions, hopes and dreams lie with the European Union are invited."
Prodi had a similar message. He was speaking in a town on the border between Slovenia and Italy that was divided after World War II and that now symbolizes the reunification of east and west.
"Today's enlargement is the fifth and the largest in the history of the union," Prodi said, "and I am convinced that it will not be the last. Other European countries and nations will decide to join our undertaking until the whole continent is unified in peace and democracy."
(compiled from wire reports)
to see RFE/RL's "EU Expands Eastward" webpage.)