Hundreds of thousands of revellers gathered in chilly weather in New York's Times Square to usher in the new decade and say goodbye to a decade marred by war, recession, terrorism and threats of environmental catastrophe.
Fireworks were set off and thousands of kilos of confetti were scattered when the gigantic New Year's Eve crystal ball dropped at midnight.
Many people wore conical party hats and 2010 glasses that blinked colourfully, and some were jumping up and down to keep warm - the National Weather Service said the temperature would be around freezing and predicted snow.
The crowds brought out heightened security. Hundreds of police officers were scattered around Times Square. Snipers were at various locations.
From fireworks over Sydney's famous bridge to balloons sent aloft in Tokyo, revellers across the globe at least temporarily shelved worries about the future to bid farewell to the Noughties.
Paris jazzed up the Eiffel Tower with a multicoloured, disco-style light display as the world basked in New Year's festivities with hopes that 2010 and beyond will bring more peace and prosperity.
Las Vegas welcomed some 315,000 revellers with fireworks from casino rooftops, a traffic-free Las Vegas Strip and toasts at nightclubs from celebrities including actress Eva Longoria and rapper 50 Cent.
Even as some major stock market indexes rose in 2009, the financial downturn hit hard, sending many industrial economies into recession, tossing millions out of work and out of their homes as foreclosures rose dramatically in some countries.
"The year that is ending has been difficult for everybody. No continent, no country, no sector has been spared," French president Nicolas Sarkozy said on national TV in a New Year's Eve address. "Even if the tests are unfinished, 2010 will be a year of renewal," he added.
German chancellor Angela Merkel warned her people that the start of the new decade would not herald immediate relief from the global economic ills.
South African president Jacob Zuma was more ebullient, saying the World Cup is set to make 2010 the country's most important year since the end of apartheid in 1994.
At midnight in Rio de Janeiro, about two million people gathered along the Copacabana beach to watch a huge fireworks display and listen to dozens of music acts and DJs.
The multitudes came mostly dressed in traditional white clothing, a nod to the Afro-Brazilian religion of Candomble but a custom followed by nearly everyone as it is thought to bring peace and good luck for the coming year.
Australian prime minister Kevin Rudd hailed events in 2009 like the inauguration of the United States' first black president, and international attempts to grapple with climate change and the global financial crisis. "The great message from 2009 is that because we've been all in this together, we've all worked together," Mr Rudd said in a New Year's message.
Australia got the some of the festivities rolling, as Sydney draped its skies with explosive bursts of crimson, purple and blue to the delight of more than one million New Year revellers near the harbour bridge.
Concerns that global warming might raise sea levels and cause other environmental problems were on the minds of some as the year ended.
Venice revellers rang in the New Year with wet feet as high tide on its archipelago peaked just before midnight to flood low-lying parts of the city - including the St Mark's Square.
Spain rang in the start of its six-month presidency of the European Union with a sound and light show illuminating Sol square in Madrid and images from the 27 member states projected onto the central post office building.
Partiers braved the cold - and a shower from sparkling cava wine bottles - in traditional style by eating 12 grapes, one with each tolling of the city hall bell.
Europe and the Americas may have partied harder than Asia. Islamic countries such as Pakistan and Afghanistan use a different calendar; China will mark the new year in February.
At Zojoji, one of Tokyo's oldest and biggest Buddhist temples, thousands of worshippers released clear, helium-filled balloons to mark the new year. Nearby Tokyo Tower twinkled with white lights, while a large 2010 sign glowed from the centre.
In Istanbul, Turkish authorities deployed some 2,000 police around Taksim Square to prevent pickpockets and the molestation of women that have marred New Year celebrations in the past.
Some officers were under cover, disguised as street vendors or "even in Santa Claus dress," Istanbul Governor Muammer Guler said.
AP