Revellers across the globe began ringing in the New Year but a Bali terror warning and a deadly shooting in Finland stoked security jitters, already high after a failed US bomb plot.
Police were on alert in many world cities after the thwarted Christmas Day attack on a US-bound plane, as party-goers from Vanuatu to Vancouver began raising glasses to 2010 and closing the door on a decade scarred by wars, terror attacks, natural disasters and financial turmoil.
The party spirit was alive and well in Sydney, where a vast display of fireworks burst into the night sky at midnight.
The US embassy in Indonesia said it had received a warning of a possible attack on the resort island of Bali, the scene of multiple bombings targeting Westerners, but local authorities denied any knowledge of such an alert.
In Finland, a lone gunman killed four people in a rampage in a shopping mall and also murdered his former girlfriend before being found dead himself.
For international troops in Afghanistan, it was business as usual, with soldiers maintaining their normal schedule of operations, after two deadly Taliban attacks claimed the lives of eight Americans and five Canadians.
In Australia, an estimated 1.5 million people crowded the harbour foreshore for the high-tech fireworks display set off from the iconic Sydney Harbour Bridge and four barges on the water.
Police minister Michael Daley urged revellers to keep a lid on their drinking.
"If you're one of these fools that can't handle their grog and likes to go out and ruin other people's nights, make yourself a new year's resolution to grow up and behave yourself and start practising that on New Year's Eve," he said.
Paris's Eiffel Tower was being transformed into a multicoloured light show, while in Berlin more than one million revellers were expected on the boulevard leading to the Brandenburg Gate, the symbol of German unity.
Celebrations in Britain centre on the London Eye, the giant wheel across the River Thames from the Houses of Parliament and Big Ben, the world's most famous clock.
A downpour of confetti marked midnight at New York's traditional mass celebration in Times Square in the heart of Manhattan.