Romanian President Traian Basescu told cheering crowds in Bucharest that they had "arrived in Europe."
In Bulgaria, people gathered in Sofia's main square to watch fireworks and sing the EU hymn. President Georgi Purvanov called it one of the most important days in the nation's history.
In Sydney, 1 million people watched fireworks over the Australian city's famous harbor, while the same numbers gathered near the Brandenburg Gate in Berlin and in New York's Times Square.
In Moscow, revelers were reportedly turned away from Red Square once a capacity crowd had filled the giant square for a midnight concert and fireworks display.
"Today we are ushering in the new year -- 2007," said Russian President Vladimir Putin in a short New Year's address to the nation. "[I wish for] fewer troubles, and that life has more meaning than just everyday problems."
Slovenia today became the first of the 10 states that joined the EU in 2004 to adopt the European currency, the euro. The Slovenian currency, the tolar, will remain in dual use with the euro for 14 days.
Also today, Germany takes over the presidency of the EU from Finland for the next six months, and of the G8 from Russia for the next year.
As always, joy came hand in hand with sorrow and death.
U.S. troops began the new year with news of the death of the 3,000th U.S. serviceman in Iraq since the March 2003 invasion.
In Thailand's capital, Bangkok, New Year's blasts killed three people and wounded more than 30 others. Thai authorities canceled New Year's festivities. It is still not known who is responsible for the attack.
Meanwhile, Pope Benedict XVI called today for nations to champion world peace and human rights, and urged people to repudiate war and violence.
The Vatican celebrates January 1 as World Peace Day.
(compiled from agency reports)