Parties have taken place across Europe as the European Union celebrated its biggest expansion in history.
Celebrations are taking place across Europe
Ten new nations have become members, turning the EU into the world's biggest trading bloc with a population of 455 million in 25 states.
A host of countries in Europe marked enlargement at midnight in Brussels (2200 GMT).
The time difference in Cyprus, Estonia, Latvia and Lithuania means celebrations have got under way an hour earlier.
Ireland, which holds the EU's revolving presidency, will host a Day of Welcomes including official ceremonies on Saturday.
The 10 leaders will join Irish Prime Minister Bertie Ahern for a flag-raising ceremony on Saturday.
The sense of history is being most keenly felt in the eight former communist states - the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Slovakia and Slovenia.
They are joining the Western club only 15 years after most of them emerged from years of Soviet domination. Some were not even separate countries until just over a decade ago.
Now all their governments have satisfied Brussels that their economies, legal systems and democracies are ready for EU membership.
"Poland's entrance into the European Union fulfils my dreams
and lifetime work," said Lech Walesa, whose Solidarity movement
toppled communism in Poland in 1989.
In ex-communist Slovakia, parliament speaker Pavol Hrusovsky echoed this sentiment.
"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 the captivity of barbed wire, the EU means the fulfilment of a dream."
The other two new members are Mediterranean islands - Malta and Cyprus - although Cypriot membership is being overshadowed by the row over Greek Cypriot rejection of a UN peace plan.
The rejection means EU membership will apply only in the Greek Cypriot part of the island, even though Greek Cypriots voted "no" in the UN referendum, while Turkish Cypriots voted to accept the plan.
Aid for Turkish Cyprus is being fast-tracked by EU ministers in recognition of their vote to accept the plan, even though EU laws will technically remain suspended there.
Security is tight in Dublin
In Dublin, security for the formal ceremonies is expected to be particularly tight.
Irish media report that police leave has been cancelled and 4,000 officers will be patrolling the streets.
Some 2,500 soldiers are also said to be on stand-by.
Reports that a "hardcore" group of up to 300 international troublemakers is planning to target Dublin have been dismissed by other protesters as scaremongering, and an attempt to deter peaceful anti-globalist, anti-capitalist protesters.
Enlargement has created enthusiasm in so-called "old" and "new" Europe alike.
"It's a new era for Europe," Belgian Prime Minister Guy Verheugen told a crowd at a ceremony close to Brussels' EU district.
Germany's Chancellor Gerhard Schroeder hailed enlargement in a speech to the lower house of parliament, the Bundestag.
"With the entry of 10 members on 1 May, the dream of many generations of Europeans will become a reality," he said.
In France, President Jacques Chirac hailed enlargement as a giant leap forward for Europe.
Prime Minister Jean-Pierre Raffarin spoke of his emotion about the occasion.
"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."
Apart from fireworks, street parties and speeches there some more unusual celebrations were planned:
- The first babies born in Lithuania after it joined were set to become stars in an 85-year-long TV documentary charting their lives
- Lithuanians were also being called on to switch on all
their lights and start fires across the country in a
bid to become the brightest country to join the EU
- Hungarian Prime Minister Peter Medgyessy was due to turn over an eight-metre hourglass at midnight to mark Hungary's accession
- Hungarians dumped unwanted belongings in a pile at a square in central Budapest
- In Estonia, 20,000 volunteers were to begin planting a million trees.