LONDON — Led by Big Ben, bells across Britain rang out Friday in a joyous cacophony, their deep bongs and tinkling tones marking the official opening day of the London Olympics.

At precisely 8:12 a.m., 12 hours before what is expected to be a spectacular Olympic opening ceremony, the bells heralded a day of celebration that has been years in the making.

Big Ben — the famous bell inside Parliament's clock tower — bonged 40 times over three minutes to ring in the games. It was joined across the country by bells and horns in churches, ships, boats, trucks and cars 12 hours before the symbolic time of 2012 British Summer Time — 8:12 p.m.

It was an exuberant display — a bit too exuberant, in the case of Olympics Secretary Jeremy Hunt, whose bell flew off its handle and almost struck a bystander during an event aboard the World War II vessel HMS Beflast.

An apologetic Hunt later posted a Twitter message: "Oops bell broke ... no one hurt but classic (hash)TwentyTwelve moment" — a reference to the BBC's satirical television comedy on the Olympics, "Twenty Twelve," in which everything that can go wrong does.

Friday's project, "All the Bells," was the idea of Turner Prize-winning artist Martin Creed, who once designed a piece of art that consisted of a light being switched on and off.

"Bells are the loudest instruments, and so I thought to do a work in public using bells, trying to make a sort of public piece of music that could be heard everywhere, you know, across the whole city and kind of across the whole country," Creed said.

Creed said the idea was to ring the bells as quickly and loudly as possible.

"There's no point in trying to be subtle about it," he said.

Olympic organizers say they believe Friday was the first time that Big Ben's 13.5 ton bell, which usually strikes the hour, has been rung outside its regular schedule since 1952, when it tolled 56 times for King George VI's funeral, once for every year of his life.

Hours before the opening ceremony, the Olympic torch was completing its 8,000-mile (12,900-kilometer) journey around the British Isles with a trip Friday down the River Thames on the royal barge Gloriana.

The torch then went into seclusion until its appearance at the opening ceremony to light the Olympic cauldron. The identity of the cauldron-lighter, and the way it will be done, is the ceremony's most closely guarded secret.

It will be the climax of director Danny Boyle's extravaganza, which is titled "Isles of Wonder" and features 10,000 volunteer performers.

Billed as a panorama of Britain's past, present and future, the opening ceremony will be seen by 60,000 spectators inside the stadium and a global television audience estimated at 1 billion. It will open with the sound of a 27-ton bell from the 442-year-old Whitechapel Bell Foundry, which also made Big Ben and Philadelphia's Liberty Bell.