Auckland in New Zealand has become the first major world city to welcome 2017.
Fireworks erupted from the 328m (1,080ft) tall Sky Tower in the city centre.
Polynesia and Pacific islands including Samoa, Tonga and Kiribati entered 2017 at 10:00 GMT.
Many cities around the world have stepped up security for New Year's Eve celebrations, after a year in which attackers drove lorries into crowds in Berlin and Nice.
In Paris, Madrid and New York, concrete barriers and heavy goods vehicles will be used to block off central squares where crowds gather to celebrate.
Thousands of extra police will be on duty in London and other cities.
Revelers in Sydney, Australia, got the party started early with a fireworks display over the Sydney Opera House and the harbour at 21:00 local time (10:00 GMT). Another display at midnight is expected to include tributes to Prince and David Bowie.
On Friday a man in Sydney was charged in connection with threats made against Sydney's New Year's Eve celebrations.
Police said he was charged with a "crimes act offence, but not a terrorist offence" and there was no continuing threat to the community.
Israel has also warned its citizens travelling in India to avoid crowds, saying there is a risk of imminent "terrorist attacks".
Celebrations in some European cities were overshadowed by security arrangements last year. In Paris and Brussels, cities that saw major terrorist attacks in 2015, official celebrations were cancelled.
In Cologne, Germany, about 1,500 extra police officers will be deployed at the city's New Year's Eve events.
It is a year since scores of women reported being assaulted, groped and in one case raped by men they described as being of Arab or North African appearance.
At the time the justice minister warned against linking the crimes to the issue of migrants and refugees, and for this year's celebration, two extremist far-right groups have been banned from holding rallies. The police cited security reasons.
Meanwhile, "leap second" will be added to the countdown to compensate for a slowdown in the Earth's rotation.
The extra second will occur as clocks strike midnight and a time of 23:59:60 GMT will be recorded, delaying 2017 momentarily.
This is required because standard time lags behind atomic clocks.