Revellers in Europe's capitals welcomed 2018 with fireworks, celebrations and street parties, while, across the Atlantic a few hours later, party-goers in New York braved frigid temperatures to watch the iconic crystal ball drop.
More than 1 million locals and tourists in New York's Times Square, who had put up with heightened security measures, had to bundle up as the temperature dropped below minus 12 degrees Celcius at midnight (0500 GMT Monday).
In Germany, tens of thousands of revellers thronged the area surrounding Berlin's landmark Brandenburg Gate to usher in 2018. Fireworks lit up the night sky at the stroke of midnight.
Paris greeted 2018 with a fireworks and pyrotechnics display at the Arc De Triomphe. Hundreds of thousands turned out along the Champs-Elysees to welcome the new year.
After several high-profile attacks in France in the past two years, authorities were taking no chances. Strict security measures were in place around the country, with some 99,000 soldiers and police officers on patrol.
In London, fireworks and cheering crowds heralded the arrival of 2018 in central London after 12 chimes from Big Ben, the bell in the iconic clock tower at the British parliament.
The clock was brought back into temporary service for Christmas and New Year's amid a four-year restoration programme at the tower, which began in August.
London Mayor Sadiq Khan, who watched the fireworks, paid tribute to London's emergency and security services.
Counterterrorism police had urged revellers to remain vigilant at outdoor New Year's celebrations in British cities, while London police warned of extra security measures.
Earlier, a rainbow firework waterfall from Sydney Harbour Bridge celebrating Australia's spectacular start to 2018 as Asia ushered in the new year.
The multi-coloured display was the finale for 1.6 million people gathered around Sydney harbour to see a 12-minute firework show that organizers said was the largest ever mounted in Sydney.
Although its New Year's celebration garnered more attention, Australia's was not the first.
Before Sydney had its big moment, New Zealanders got to see a five-minute fireworks show from Sky Tower in Auckland.
However, the very first celebrations were on the Pacific nations of Samoa, Tonga and Kiribati. People there celebrated the new year with traditional juggling and dancing.
Thailand resumed its colourful new year celebrations and music as the country ended a year-long mourning for late king Bhumibol Adulyadej who died in October 2016.
Thailand marks the beginning of the year on a religious note as people flocked to temples across the country to join Buddhist chanting overnight, while others visited temples early Monday to give alms to monks for an auspicious beginning of the year.
With the start of the Year of the Dog still six weeks away, China took a more low-key approach to the arrival of 2018.
Large cities saw many people gather to celebrate the new year, and hotels offered New Year's Eve parties to Western guests, but there were no official firework displays to mark what is still a public holiday.
