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Pope Benedict XVI issued a New Year's Day call for tolerance after massive celebrations worldwide ushered in a fresh decade under heavy security and amid warnings of more extremist plots.
The pope's comments came after New Yorkers partied while being protected by rooftop snipers in Times Square following the Christmas Day attempt by a Nigerian suspect to blow up a US airliner bound for Detroit.
British Prime Minister Gordon Brown said the failed plane bombing showed that terrorism remained a "very real" global threat as the world enters a new ,decade eight years after 9/11.
In his traditional New Year's Day mass Friday, Benedict called for respect and tolerance.
"Respect others, regardless of their skin colour, nationality, language, religion," the pope said.
He also spoke against war and violence, deploring that too often the faces of children were "sunken by hunger and illness, disfigured by pain and hopelessness".
Before such defenceless humans, "all the false justifications for war and violence fall down".
For the Archbishop of Canterbury Rowan Williams, the past decade had been a "terrible and gruelling 10 years in all kinds of ways."
But the leader of the world's 77 million Anglicans added that people should not give up on the "hope for change" or "shrug our shoulders and lower our expectations".
In the world's major cities, millions turned out for parties to bid farewell to a decade that saw the September 11, 2001 attacks, the launch of wars in Afghanistan and Iraq, and a disastrous economic crisis.
But security was tight across the globe after the airliner bomb plot.
Thousands of police officers deployed in New York, backed by surveillance cameras, rooftop snipers and devices able to detect radiation or biological agents.
Partygoers in Times Square were not allowed to carry backpacks or alcohol.
On Wednesday, police had ordered the brief evacuation of Times Square and investigated what they said was a suspicious van -- which later proved to be harmless.
"We assume here that New York is the number one terrorist target in America," city police Commissioner Raymond Kelly said.
The British prime minister, in an article on his Downing Street office's website, spoke of extremist threats.
"The new decade is starting as the last began -- with Al-Qaeda creating a climate of fear," Brown wrote, saying the failed bombing had "exposed an evolving terrorist threat".
"The failed attack in Detroit on Christmas Day reminds us of a deeper reality: that almost 10 years after September 11th international terrorism is still a very real threat."
The New Year, of course, had its lighter moments as well.
Russians used to stern-faced messages from their leaders were treated to a surprise cartoon on state television caricaturing President Dmitry Medvedev and Prime Minister Vladimir Putin.
Television cartoons of Russian leaders have been a virtual taboo over the last half decade.
Celebrations at iconic European sites, including the Eiffel Tower in Paris and the London Eye across from Big Ben, saw tens of thousands of revelers joyously welcome the New Year.
Spain had an extra reason to party, with the country taking over the EU presidency for six months at midnight.
"This is the best street party in the world. Now I am going to work on my first hangover of 2010," said Gerry Shalloe, a 32-year-old English teacher from Ireland who lives in Madrid.
Auckland, New Zealand, was the first major city to see in the New Year with a burst of fireworks, and in Australia about 1.5 million people watched a pyrotechnic display from the iconic Sydney Harbour Bridge.
There were also hopeful messages, with South African President Jacob Zuma using his New Year comments to rally for unity for the 2010 football World Cup -- the first ever to be held in Africa.
But in Afghanistan, where Western powers have been fighting the Taliban for most of the past decade, US-led soldiers were on alert after two militant attacks killed eight Americans and five Canadians.
In the Philippines, at least five people were killed and about 600 injured in revelry involving fireworks and guns, which draw on ancient superstitions that noise drives away evil spirits.
