Hundreds of thousands of people were in central London to celebrate the beginning of a new century as Big Ben chimed midnight.
In pictures: London parties

In Edinburgh, a magnificent firework display lit up the Scottish skies.
In pictures: Scotland celebrates

The streets of Cardiff buzzed with life amid the excitement of the big night.
In Pictures: Wales welcomes Y2K

More fireworks - and no less stunning than anywhere else - brought in the New Year in Belfast.
In Pictures: New Year in Northern Ireland

Across the UK, some of the music world's biggest stars - including The Corrs - wowed the huge crowds.
In Pictures: Pop goes the 20th century

The parties went on late into the night - not least in Northumberland, where revellers marked the new millennium with the local tradition of carrying blazing tar barrels on their heads.
In Pictures: UK united in celebration