The new 25-member European Union has marked its historic expansion with celebrations across the new bloc.
The EU is now the world's largest trading bloc
The 15 old members welcomed in Cyprus, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia and Slovenia at midnight.
The most high-profile festivities took place in Ireland, current holder of the rotating EU presidency.
Irish Prime Minister Bertie Ahern welcomed the new members and hailed a "day of hope and opportunity".
In bright spring sunshine, the leaders of the new member states were welcomed in a simple ceremony in the grounds of the Irish president's official residence by their counterparts from the existing 15 members.
They watched as young people from all 25 countries presented their national flags, which were then raised together alongside the EU flag as a mass choir sang the EU anthem, Beethoven's Ode to Joy.
Mr Ahern spoke of the progress that Europe had made over the past decades, saying it had moved from war to peace.
He went on: "From hatred there is now respect, from division there is union, and from dictatorship and oppression there is democracy. "
But he also made reference to the challenges ahead for the enlarged club - notably the need to find agreement on the thorny issue of a constitution, and to narrow the now even more pronounced wealth gap between members.
"There is indeed much work to be done," he said.
Ireland mounted its biggest security operation since Pope John Paul II visited in 1979 in preparation for the celebrations.
In Saturday evening, riot police used water cannon to break up hundreds of anti-capitalist protesters in Dublin, but no serious incidents were reported.
Joy and uncertainty
With a population of 455m, the EU now is the world's biggest trading bloc.
Hundreds of thousands packed city squares in the newcomer states to watch fireworks and hear Beethoven's Ode to Joy - the EU's official anthem.
The BBC's Tim Franks says some enthusiasts are describing the enlargement as a millennial event, comparable to the creation of great empires.
This is a hugely significant day for Europe, our correspondent says, but it is nowhere near the end of the story.
In the existing member states, there is more uncertainty over immigration, over the new balance of forces within the EU and over whom the club should admit next.
For the newcomers, there are concerns about price hikes without commensurate salary increases.
There is also disappointment that established members have placed restrictions of up to seven years on freedom of movement for workers from the relatively poor east into the west.
Eight of the new members are former communist states, joining the Western club only 15 years after most of them emerged from years of Soviet domination. Some did not even regain their independence until just over a decade ago.
'No war again'
The other two new members - Malta and Cyprus - are Mediterranean islands.
But Cypriot membership is being overshadowed by the exclusion of the island's Turkish Cypriot part, following an inconclusive referendum on reunification a week ago.
One of the fathers of European reunification, former German Chancellor Helmut Kohl, spoke through tears when he addressed thousands at a ceremony in the German town of Zittau, which borders both Poland and the Czech Republic.
"The message is there will never again be war in Europe," Mr Kohl said.
Marek Wos, a 40-year-old Polish businessman attending the celebrations in Warsaw, said it was a good day for his country.
"We will no longer be second-class people from a second-class country," he said.