In the "old Europe" of the EU's 15 western states, there were fewer celebrations, although fireworks lit up the evening sky in Dublin, in a show organised by the current EU president, Ireland.
Thousands braved a chilly twilight to enjoy the pyrotechnics, launched from 25 barges - one for each of the EU member states - along Dublin's River Liffey.
People in the Baltic states burst into song to celebrate joining the EU, 14 years after singing their way out of the Soviet Union after the fall of the Berlin Wall in 1989.
"Latvia has prepared for the EU, Latvia has strived and argued and achieved a result. Now it is time to celebrate," President Vaira-Vike Freiberga said to a crowd of 30,000 new Europeans gathered for a concert on the banks of Riga's Daugava river.
The 10 countries joining the EU are Cyprus, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia and Slovenia.
Spectacular pyrotechnics light up
the Hungarian capital Budapest
The people of Lithuania switched on all their lights and started fires across the country late on Friday in an attempt to become the brightest country to join the EU. A US satellite was standing by to photograph the event from space.
In Prague, tens of thousands of people gathered in the city's main square for a concert of several hours, capped by a fireworks display at midnight after the Czech flag and the starry blue EU flag were raised and the anthems of both were played.
The most sour note of the day came from Euro-sceptic Czech President Vaclav Klaus, who has warned that new EU states will be losing their sovereignty.
"I want to welcome both those who look forward to EU entry and those who see it with definite qualms. But I want to warn that they could be disillusioned."
Dream come true
In Poland, however, former president and Solidarity trade union leader Lech Walesa described his country's entry into the EU as a dream come true.
"I fought for our country to recover everything it lost under communism and the Soviets ... and now my struggle is over. My ship has come to port," Walesa was quoted as saying in the Zycie Warszawy newspaper.
EU Commission president Romano Prodi welcomed the enlargement in a midnight ceremony in a town on the Italian-Slovene border that was divided after the second world war and now symbolises the reunification of East and West.
"Today's enlargement is the fifth and the largest in the history of the union and I am convinced that it will not be the last," Prodi said.