Celebrations have begun across Europe to mark the enlargement of the European Union to a 25-nation economic and political bloc of 450 million people.
EU enlargement took effect last night when 10 new nations officially became members.
Leaders from all 25 states will gather for a ceremony at Aras an Uachtarain tonight.
Eight former Warsaw Pact members Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Slovakia and Slovenia join Cyprus and Malta to raise the EU's membership from 15 to 25 and turn the bloc into the world's largest free trade area.
Star-studded blue EU flags went up and fireworks lit up the sky at midnight across eastern and central Europe, marking the largest single enlargement of the European Union since its inception.
Thousands of people turned up in Sandymount last night for a spectacular fireworks display to mark the accession.
"Poland's entrance into the European Union fulfills my dreams and lifetime work," said Mr Lech Walesa, whose Solidarity movement toppled communism in Poland in 1989, said today.
The EU faces profound change as it tries to integrate poorer east Europeans and stay manageable with its border posts moving 1,000 kilometres east, to the frontiers of Belarus, Russia and Ukraine.
"May 1 will be a milestone in the history of Europe," EU Enlargement Commissioner Mr Guenter Verheugen said in Warsaw. "It is Europe's response to the end of the Cold War and an opportunity to heal the wounds of the past, wounds of war and dictatorship."
Scheduled alongside the official pomp were some unorthodox celebrations.
Lithuanians switched on lights countrywide to make the land glow on satellite pictures; Hungarians dumped unwanted belongings in a pile at a central Budapest square.
In Estonia, 20,000 volunteers started planting a million trees; Czechs and their German neighbours created rainbows bridging "West" and "East" using water cannon and floodlights.
Earlier last night, several hundred protestors participated in a Critical Mass slow cycle through the streets of Dublin. Protests and carnival events are planned for tomorrow to mark May Day and the accession of the new EU states at the ceremony in Aras an Uachtarain.
Dublin has seen unprecedented security with 5,000 gardai and 2,500 soldiers drafted in to police the event.