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.
Last year, celebrations in Europe were also overshadowed by security arrangements. In Paris and Brussels, cities that saw major terrorist attacks in 2015, official celebrations were cancelled.
A “leap second” will be added to the countdown in countries in the GMT timezone 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.