Big Ben has chimed non-stop for three minutes to ring in the Olympic Games.
The London landmark was joined by hundreds of churches and other organisations across the nation as bell ringers greeted the official start of London 2012.
The hour bell of the landmark Palace of Westminster clock began chiming at 8.12am. It pealed 40 times over the following three minutes. Special permission had to be gained for the hour bell at the Palace of Westminster to be allowed to toll out of its regular sequence.
It is believed to be the first time that the strike of Big Ben has been rung outside its normal schedule since February 15 1952, when it tolled every minute for 56 strokes for the funeral of King George VI.
The bells at the National Assembly for Wales, Stormont in Northern Ireland and the Scottish Parliament also rang so that all four Parliaments chimed in unison at 8.12am, London 2012 said.
The cacophony of sound is for the All The Bells event, the brainchild of Turner Prize-winning artist and musician Martin Creed, who is encouraging everyone in the UK to ring a bell.
His piece of conceptual art, which has the official title Work No. 1197: All the bells in a country rung as quickly and as loudly as possible for three minutes, was devised as part of the London 2012 Festival.
The ringing of Big Ben was attended by the Speaker of the House of Commons, John Bercow MP, Lord Tony Hall, chair of the London 2012 Cultural Olympiad Board and bell ringers from a variety of organisations.
The bell on board the royal rowbarge Gloriana also pealed for three minutes as it processed down the Thames, carrying the Olympic flame and a ceremonial cauldron.
HMS Belfast fired its canons in a countdown to 8.12am as 300 children, sea cadets, brownies and Town Criers gather on board to ring an assortment of bells. Others taking part included The Royal Navy and the Royal Fleet Auxiliary, the British Army, the RAF, the National Trust, the National Theatre, the Ancient and Honourable Guild of Town Criers, the Royal British Legion and the Women's Institute.