Celebrations will move to Asia, the Middle East, Africa, Europe and finally the Americas, with dazzling light shows bidding farewell to 2017
Fireworks lit up the sky above Sydney Harbour a few hours before midnight Sunday, kicking off the city's New Year's celebrations.
Around one million people gathered to watch the festivities. Security was tight, but officials said there was no particular alert.
During the midnight fireworks display, a rainbow waterfall off the Sydney Harbour Bridge wowed onlookers.
In Melbourne, here's how the sky lit up
Three hours ahead of the Philippines, it’s now officially 2018 here in Australia.
Warm regards from the world’s most liveable city, Melbourne! #UNTVRepaso2017@UNTVNewsRescue pic.twitter.com/7VMzZEiu3h— Stacey (@basicallystacey) December 31, 2017
"Shooting stars" were fired from the rooftops of skyscrapers in a 10-minute musical fireworks display.
Some 500 couples wed in a mass ceremony sponsored by the government, and festivals and bazaars will be held on main roads and at tourist sites.
The city is replacing its main midnight fireworks with a laser show on the world's tallest tower, the 828-metre Burj Khalifa. Follow our live coverage
New Year celebrations kicked off in the Philippines, with households setting off fireworks. Hours before midnight, authorities had already reported that at least 86 people had been injured by celebratory firecrackers in the Philippines, which has some of the most raucous New Year's celebrations in Asia.
In Metro Manila, revellers were entertained by a New Year's eve countdown party in Eastwood mall.
Fireworks lit up the Singapore skyline to greet 2018.
Major boulevards and squares will be decked out to welcome the new year, with fireworks to light up 36 key sites.
Special tents will be set up at the Brandenburg Gate to care for women victims of sexual harassment or those who feel threatened, following mass assaults by migrant groups on women in Cologne two years ago. In Cologne itself, 1,400 police will be mobilised, street lighting will be improved and more video cameras installed.
Hundreds of thousands are expected to line the Champs-Elysees for a light show and fireworks at the Arc de Triomphe. Nearly 140,000 police, gendarmes and soldiers will be deployed nationwide to guard against the jihadist threat. But with no major attack in France since mid-2016 the atmosphere was noticeably more festive than in the past two years.
Millions will gather on Copacabana beach to watch the fireworks, with many wearing white, the traditional colour to usher in the new year.
Tens of thousands of New Zealanders took to streets and beaches, becoming among the first in the world to usher in 2018.
As the new year dawned in this southern hemisphere nation, fireworks boomed and crackled above city centers and harbors, and party-goers sang, hugged, danced and kissed.
In Auckland, New Zealand's biggest city, tens of thousands gathered around Sky Tower as five minutes of nonstop pyrotechnics exploded from the structure's upper decks.
But on nearby Waiheke Island, 30 kilometres away, authorities canceled the planned fireworks display because of drought conditions and low water supplies for firefighters.
Thousands of people filled the streets near Seoul's City Hall for a traditional bell-tolling ceremony to usher in the new year.
The group of dignitaries picked to ring the old Bosingak bell at midnight includes Soohorang and Bandabi - the tiger and bear mascots for the Pyeongchang Winter Games and Paralympics in February and March.
Tens of thousands of people flocked to eastern coastal areas, including Gangneung, the seaside city that will host the Olympic skating and hockey events, to watch the sun rise on 2018. In Seoul, fireworks li the sky over the Lotte World Tower, a 123-floor building, after midnight.
Security was tight in the southern Indian city of Bangalore to prevent a repeat of incidents of alleged groping and molestation of several women during last year's New Year's Eve celebrations.
Sunil Kumar, the city's police commissioner, said at least 15,000 police officers were on duty and were being aided by drones and additional closed-circuit television cameras.
Last year, police first denied that any sexual harassment had taken place during the celebrations in Bangalore, India's information technology hub. But later, police detained at least six men after several video clips of women being attacked by groups of men spread on social media.
Huge crowd of revelers expected in Times Square will brace for what could be one of the coldest New Year's Eve ball drops on record.
Brutal weather has iced plans for scores of events in the Northeast from New Year's Eve through New Year's Day, but not in New York City, where people will start gathering in Times Square up to nine hours before the famous ball drop.
Those willing to brave the cold in Beijing will join a countdown at the tower at Yongdingmen Gate, a rebuilt version of the Ming dynasty-era landmark gate at the southern end of the city's north-south axis.
Bells will be rung and prayers offered at temples in Beijing, but the Gregorian calendar's New Year's celebrations are typically muted in China compared to the Lunar New Year, or Spring Festival, a time of fireworks, feasts and family reunions.
Authorities throughout China are also on high alert for stampedes or terror attacks at large public gatherings. Police in the central city of Zhengzhou are putting 3,500 officers on duty across the city while residents gather to watch a light show and cultural performance in a public square.
The official Xinhua News Agency reported that President Xi Jinping sent a New Year's greeting to his Russian counterpart, Vladimir Putin, saying Beijing is ready to boost cooperation with Russia in 2018.
Many Japanese are celebrating the arrival of the Year of the Dog in the traditional way of praying for peace and good fortune at neighbourhood Shinto shrines, and eating New Year's food such as noodles, shrimp and sweet black beans.
Barbecued beef and octopus dumpling stalls were out at Tokyo's Zojoji Temple, where people take turns striking the giant bell 108 times at midnight, an annual practice repeated at other Buddhist temples throughout Japan.
North Korea's nuclear and missile programs cast a shadow over Japan's hopes for peace, said 33-year-old cab driver Masaru Eguchi, who was ready to be busy all night shuttling shrine visitors.
"The world situation has grown so complex," Eguchi said, adding that he also worried about possible terrorism targeting Japan. "I feel this very abstracted sense of uncertainty, although I really have no idea what might happen."
Security measures were ramped up across Turkey, which was hit by a New Year's attack a year ago that killed dozens.
In Istanbul alone, 37,000 officers were on duty, with multiple streets closed to traffic and large vehicles barred from entering certain districts. Several New Year's Eve street parties were canceled for security reasons.
Early on January 1, 2017, an assailant shot his way into Istanbul's Reina nightclub, where hundreds were celebrating New Year's. Thirty-nine people were killed - mostly foreigners - and 79 wounded. Daesh claimed responsibility.
On Sunday, some 100 people gathered outside the nightclub to remember the victims of the attack.
Romanians prepared to usher in a new year in which the focus is expected to be an anti-corruption fight as the government seeks to push through legislation that critics say will make it harder to punish high-level graft.
Television stations broadcast live from supermarkets full of last-minute shoppers, while beauty salons reported full bookings as revelers geared up for traditional celebrations of copious meals that can run to hundreds of dollars.
Others meditated and prayed at Orthodox churches and monasteries. In rural eastern Romania, villagers danced traditional pantomime-like jigs to welcome the new year, wrapping themselves in bear furs or dressing as horses.
In his New Year message, President Klaus Iohannis praised Romanians who staged the largest protests since the end of communism.
Tens of thousands of revelers will ring in the new year in Las Vegas under the close eye of law enforcement just three months after the deadliest mass shooting in modern US history.
Tourism officials expect about 330,000 people to come to Las Vegas for the festivities, which are anchored by a roughly eight-minute fireworks display at the top of seven casino-hotels.
Acts including Bruno Mars, Britney Spears, Celine Dion and the Foo Fighters will keep partiers entertained before and after midnight at properties across Sin City.
The Las Vegas Metropolitan Police Department will have every officer working Sunday, while the Nevada National Guard is activating about 350 soldiers and airmen.
The federal government also is sending dozens of personnel to assist with intelligence and other efforts.
Runners take part in the 41st Vienna New Year's Eve run on the Ringstrasse in Vienna.
The circular course takes place over 5,350 metres in Vienna's center and is attended by 4,500 runners.
In Indonesia's Bali Island, groups of Balinese are taking part in a traditional dance for a cultural parade to mark the New Year.
Up Helly Aa vikings from the Shetland Islands hold lit torches during the annual torchlight procession to mark the start of Hogmanay (New Year) celebrations in Edinburgh, Scotland
Tens of thousands of Parisians and tourists headed to the Champs-Elysees to attend a firework show at Napoleon's Arc de Triomphe monument, at the end of the famous avenue with its lines of trees sparkling with lights.
New Year's Eve celebrations were placed under high security in France, which has been hit by a series of attacks by Islamic extremists in recent years.
French Interior ministry said 100,000 police officers and soldiers and 40,000 rescuers have been deployed across the country - including 2,500 on the Champs-Elysees.
Bidding 2017 farewell, Pope Francis has decried wars, injustices and environmental decay which he says have "ruined" the year.
Francis on Sunday presided at a New Year's Eve prayer service in St. Peter's Basilica, a traditional occasion to say thanks in each year's last hours.
He says God gave to us a 2017 "whole and sound," but that "we humans in many ways ruined and hurt it with works of death, lies and injustices."
But, he added, "gratitude prevails" thanks to those "cooperating silently for the common good."
In keeping with past practice, the pope on New Year's Day will celebrate Mass dedicated to the theme of world peace.
With input from agencies