Giant Olympic rings have been launched on the River Thames in London to mark 150 days until the start of the 2012 Games.

The rings – 25 metres wide and 11 metres high – travelled on a barge along the Thames, which runs through the centre of the British capital, passing such landmarks as Tower Bridge and Canary Wharf.

London Mayor Boris Johnson also announced a program of free cultural events to accompany the Games.

Johnson said: “2012 is one of the most thrilling years in our city’s history and as we welcome the world, we have an unprecedented opportunity to showcase and strengthen London’s outstanding culture and creativity, which are a hugely important part of our economy.

“From museums to theatre, and music to design, we have a reputation that is second to none and London’s finest artists and organisations are producing an exceptional range of brand new cultural events that will add to the excitement of hosting the Games.

“We’re creating the biggest festival of outdoor arts ever to be seen in the capital, as well as fantastic new work that will throw new light on some of our city’s lesser-known landmarks and hidden gems.”

The rings on display on Tuesday will be back on the Thames at the start of the Olympics, which get under way on July 27.