Millions of revellers welcomed in the New Year with celebrations in towns and cities across the UK.
An estimated 200,000 people gathered in central London to usher in 2006, said police, and about 100,000 in Edinburgh.
The skies above both capital cities were lit up by a dazzling array of fireworks as midnight arrived.
A 24-hour Tube strike had threatened to dampen celebrations in London, but transport bosses said the action had little impact.
London Mayor Ken Livingstone said the celebrations "showed that no one should under-estimate the sheer determination of Londoners to enjoy a good party".
He praised the police, emergency and transport staff and said the Tube strike had hardly materialised.
On New Year's Day the celebrations continued in central London as thousands gathered for the 20th annual parade.
More than 10,000 performers from across the world are joining the route from Parliament Square to Piccadilly.
London's New Year's Eve celebrations began with lights going on at City Hall and the London Eye.
Images of sporting heroes - including Andrew Flintoff MBE and Dame Ellen MacArthur - were projected onto buildings.
At midnight, a 10-minute volley of fireworks was sent into the sky against the backdrop of the London Eye and Big Ben, to cheers from delighted spectators.
Thousands of people descended on the Embankment and surrounding areas to watch the spectacular display.
And in Trafalgar Square partygoers linked arms to sing Auld Lang Syne under Nelson's Column.
"It's been a tough year with the London bombings - bring on 2006," said policeman Sam Azouelos, 26, from north London, who was in the square.
"I think with the New Year, people have the chance to put those times behind them and just look forward to the future instead."
Paul Petrie, 38, a kitchen fitter, and 41-year-old project planner Liz Selbie were visiting from Raunds in Northamptonshire.
"We've always wanted to do it and it's been fantastic," Mr Petrie said.
'Better and better'
At Edinburgh's celebrations, about 100,000 people filled the city's main thoroughfare Princes Street for the world-famous Hogmanay Party.
Revellers were entertained by acts on four different stages and were treated to a wide variety of sounds, from traditional Scottish music to pop.
A 10-second countdown gave way to the sound of Big Ben ringing out on the loudspeakers at midnight, and as 2005 turned into 2006, eight tons of fireworks lit up the night sky.
The extravaganza marked the highlight of Edinburgh's four-day Hogmanay celebrations.
"I think every year it just gets better and better," said Lesley Hinds, the Scottish capital's Lord Provost.
The ceremonial mayor said the city had been in the spotlight in July, when G8 leaders gathered at the nearby Gleneagles resort.
"After the G8 summit, where we projected Edinburgh all over the world, people want to come the city because they know it's the best party in the world," she told the BBC.
Tube stations shut
In other cities, crowds of thousands gathered in Cardiff; Leeds hosted fireworks displays across the city centre, and in Newcastle Gateshead early evening fireworks closed the city's winter festival.
In London the celebrations passed calmly, with police saying 69 people had been arrested - mainly for disorder and drink-related offences.
But there were 35 stabbings across London reported by the ambulance service on a night it dealt with a record number of emergency 999 calls.
Between midnight and 4am the service dealt with 1,444 calls, up 4% on the same period last year.
Deputy director of operations Russell Smith said: "We are horrified that there have been so many stabbings on what is an evening of celebration for most people".
He said the majority of calls had been alcohol-related.
About 30 or so of the 275 London Underground stations were closed due to lack of staff after the strike began at midday on New Year's Eve, transport managers said.
But British Transport Police said the evening had passed "very well indeed" with no reports of overcrowding.