Hundreds of thousands of revellers have welcomed in the new year at open-air parties across the UK.
About 80,000 people attended the annual Hogmanay party in Edinburgh, where the crowd erupted in cheers as the clock struck midnight.
An estimated 250,000 people watched the eight-minute firework display at the London Eye, which was set to music for the first time.
Events were held in various other cities, including Glasgow and Cardiff.
Edinburgh's Hogmanay festival lasts four days and got under way on Thursday with a torchlight procession.
The ticket-only main event on New Year's Eve included fireworks, traditional Scottish dancing and an open-air concert starring Biffy Clyro, The Charlatans and Billy Bragg.
Bragg, who stayed on with his family to watch Biffy Clyro and the fireworks after his performance, said Scotland had "kind of invented new year get-togethers", so Edinburgh was where he wanted to be.
Biffy Clyro, whose hit Many of Horrors has become a number one single for X Factor winner Matt Cardle, took to the stage topless and wowed the crowd with a volley of their anthems.
The Scottish rockers' frontman Simon Neil said: "There's no better way to end the year."
Irina Rusina, originally from Siberia, said it was her first time in Scotland and she was loving Edinburgh.
"I'm completely enjoying myself," she said. "It is a lot warmer but much more crowded than it is in Siberia."
Edinburgh Hogmanay director Peter Irvine said Edinburgh was now "on the map" for new year celebrations.
"This was a truly great night. Nothing went wrong, from the weather to the crowds, who are here from all over the world. They were incredibly good-natured, no aggravation."
Around 330 police officers patrolled Princes Street and the surrounding area, and one person was arrested for a "minor disorder".
About 5,000 people packed into Glasgow's George Square to see in 2011 with one of Scotland's biggest outdoor ceilidhs.
Crowds also gathered in central London for the free annual fireworks, which were launched from barges on the Thames and the London Eye.
Showers of brightly coloured fireworks shot into the sky to a soundtrack including Queen's We Will Rock You, The Beatles' Lucy in the Sky with Diamonds, and Blur's Song 2.
Erin Wilkins, 26, and Emad Tehrani, 30, had travelled from Sydney to see in the new year in London.
Ms Wilkins said: "We've been queuing up here since midday and I think these are the best fireworks I have ever seen."
Some 3,000 police officers were on duty, and the Metropolitan Police said 77 arrests had been made during the celebrations. The majority (24) were for public order offences, while 16 revellers were arrested for assault and five for sexual offences.
The celebrations will continue in London on Saturday with 500,000 people expected to flock to Westminster for the 25th New Year's Day parade.
More than 10,000 musicians, cheerleaders and performers, from 20 London boroughs and 20 countries, will take part in the parade. A special message from the Queen will be read out to start proceedings.
In Wales on New Year's Eve, crowds attended the free Calennig celebrations at Cardiff Civic Centre, which featured a live music concert with Dr and The Medics and T.
The event included a fire show and funfair, as well as midnight skating at an open-air ice rink.
Across the UK party organisers were no doubt relieved the freezing conditions of the past few weeks were replaced by milder weather, with temperatures of 4-5C in Edinburgh, Glasgow and London.
The BBC Weather Centre said it had been a dull and misty end to 2010.