Large crowds lined the banks of the River Thames in London and the streets of Edinburgh as the UK welcomed the start of the New Year.
Along the Thames, up to 250,000 people watched a major fireworks display which saw the night sky illuminated with pyrotechnics to round-off a momentous year.
Revellers were able to watch clips from the 2012 Olympic Games during an 11-minute firework display set around the London Eye. Millions more watched it all on television.
In Edinburgh, tens of thousands of people descended on the streets of Scotland's capital for the world-famous Hogmanay celebrations.
Around 75,000 party-goers counted down the last 10 seconds of 2012 before joining in a mass rendition of Auld Lang Syne after the clock struck midnight.
Scottish rockers Simple Minds headlined the Hogmanay concert and street party.
The View, Bwani Junction, Reverend and the Makers and The OK Social Club also played, and fireworks were set off to mark the start of 2013.
Pete Irvine, artistic director of the Hogmanay celebrations, said: "I think it's going really well this year. It's really, really busy here tonight - probably busier than we've seen it in some time because the weather is so good."
Following their hard work during the London 2012 Games, more than 150 of the mayor's "Team London Ambassadors" volunteered at the Thames event to help ensure it was a success.
They gave out maps, helped direct people to the viewing areas, and provided guidance and advice on getting home safely.
As daylight broke in London, some 200 clean-up staff cleared 160 tonnes of rubbish left by revellers in Westminster.
The workers took just six hours to clear the streets ahead of today's New Year's Day parade in the city centre.
The Metropolitan Police said 96 arrests were made during Monday night's celebrations, with drunkenness, public disorder and assault the most common offences.
More than 3,500 police supported the organisers and stewards of the event, working alongside colleagues from the British Transport Police and other emergency services to keep revellers safe.
Chief Inspector John Williams said: "This year saw the viewing areas fill up earlier than ever before with thousands of revellers coming to see in the new year London-style.
"Officers worked hard, alongside the stewards, in very large crowds to keep people safe in what is a challenging policing environment, helping to ensure revellers and visitors alike could make their way home at the end of the night."
The Mayor of London, Boris Johnson, said: "What an amazing end to an incredible year.
"Watched by hundreds of thousands in the capital and millions around the globe, London has proved, yet again, that it can pull off spectacular world-class events in style."
Today's New Year's Day parade will see a procession of Olympic Games Makers, drummers, stilt walkers and Bolivian dancers greet 2013 as they snake their way through central London.
The New Year celebrations got under way in Edinburgh as early as Sunday night, when a torchlight parade was held.
A record crowd estimated at 35,000, including 7,000 torch carriers, were led by the massed pipes and drums and the Up Helly Aa' Vikings from Shetland.
The procession made its way through the city streets to the burning of the effigy of a Viking boat and a spectacular Son et Lumiere display on Calton Hill.