Crowds are gathering on the banks of the River Thames and in city centres across the UK as they wait to count in the New Year.
London revellers are being watched over by every available officer at the Metropolitan Police's disposal, including hundreds of firearms personnel.
As many as 3,000 officers are working across the centre of the capital, as cities across the world heighten security in the wake of the Paris terror attacks.
Many braved the intermittent spells of rain awaiting a midnight display featuring around 12,000 fireworks.
Other global cities have already beckoned in the New Year, including Dubai, where celebrations were tempered by a huge blaze in one of its tallest buildings.
In Moscow, the arrival of the New Year was greeted by silence - as the firework display was five minutes late.
But in Brussels, the midnight display was cancelled because of threats of an extremist attack.
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.
Metropolitan Police spokeswoman Superintendent Jo Edwards said: "We're mindful of what's going on in other cities and across the world, we're linked into the intelligence services, but there is no specific intelligence about the event in London tonight.
"We have every confidence our policing plan is as good as it can be. We've been policing at the threat level of severe for some time now, we are very experienced in London policing large-scale events.
"People who are in London tonight will probably see more police officers visible, including firearm officers, but that's been the case since those dreadful attacks in Paris.
"We want people to be reassured, not frightened by the number of police officers they may see."
She also addressed concerns that the Met's focus on policing the New Year's fireworks on the Thames would reduce coverage of other boroughs.
"Business as usual policing will carry on across the boroughs - there will be policing plans in place as appropriate across all the boroughs," she added.
The Metropolitan Police said it was working closely with the mayor's office, British Transport Police (BTP), Westminster City Council and others to ensure the event ran smoothly.
The mayor's fireworks display was ticketed for the second year and has sold out.
On New Year's Day, more than 8,500 performers representing 20 countries will take part in an annual parade through London.
The event, which includes marching bands, cheerleaders, clowns and acrobats, kicks off in Piccadilly and finishes in Parliament Square, taking in Lower Regent Street, Pall Mall, Trafalgar Square and Whitehall.