Crowds gathered on the banks of the Thames and in city centres across the UK to see spectacular fireworks displays ring in the start of 2016.
In central London, around 3,000 officers, including firearms police, were deployed to watch over New Year revellers at a time of heightened global security in the wake of the Paris terror attacks. The Met Police today described the evening as "successful".
Tens of thousands lined banks and bridges along the Thames, braving intermittent spells of rain ahead of a midnight display featuring around 12,000 fireworks which produced 50,000 projectiles.
There were a total of 48 arrests overnight, including four for drug possession, three for carrying offensive or bladed weapons and five for assaults on police officers. Six people were also detained for public order offences, 11 for assault and one for sexual assault, the Met said.
The London Ambulance Service reported fewer casualties overnight than last year, with 55 patients needing hospital treatment, compared to 66 on the same night in 2014.
In total, the service took 500 emergency calls over peak hours - double the number it usually receives - with the majority being made between 2am-3am on New Year's Day.
The Edinburgh Hogmanay celebrations also saw several firework displays above the city's castle, as well as a performance from Scottish rockers Biffy Clyro.
However, a similar display in Brussels was cancelled due to safety concerns and in Munich two railway stations were closed due to a "serious imminent threat".
German media reported that authorities were tipped off by a foreign intelligence service that Islamic State (IS) was planning attacks with five to seven suicide bombers.
In Dubai, the New Year celebrations were tempered by a huge blaze in one of its tallest buildings.
The arrival of the New Year was greeted by silence in Moscow as the fireworks display was five minutes late.
The Met confirmed New Year's Eve was one of two events - along with Notting Hill Carnival - in which there was no holiday entitlement, meaning all available staff were called upon.
Met spokeswoman Superintendent Jo Edwards said the extra officers were for reassurance and the event was "successful".
She said: "Our officers worked alongside stewards to support their operation and help people to get in and out of the event area safely and efficiently.
Today presents another test for the Met Police. More than 8,500 performers representing 20 countries are taking part in the annual New Year's Day parade through London, again amid a large security operation.
The event, which includes marching bands, cheerleaders, clowns and acrobats, kicked off in Piccadilly and was due to finish in Parliament Square, taking in Lower Regent Street, Pall Mall, Trafalgar Square and Whitehall.