Olympic rings unveiled on London's Tower Bridge - video Reuters

Giant Olympic rings have been installed over the Thames on Tower Bridge to mark the countdown to the London 2012 Games.

The London 2012 chairman Lord Coe and the mayor Boris Johnson were among the VIPs who watched as the rings – 25 metres wide and 11.5 metres tall – were lowered into place on the landmark.

The rings, which weigh three tonnes, cost £259,817 to produce and installing them is estimated to have cost £53,000. All costs have been met from the £32m Look and Celebration budget, which will be used to back free cultural events across London.

Lord Coe said: "The Olympic rings are an iconic symbol, inspiring athletes and uniting people around the world. To athletes they represent the culmination of thousands of hours of training and reaching the highest level in sport.

"With one month to go to the Olympic Games opening ceremony, these spectacular rings on one of London's most famous landmarks will excite and inspire residents and visitors in the capital."

A light show, complete with beams of changing colours and intensity, will bring the rings to life at night. It will put into action Tower Bridge's new lighting system, involving 1,800 special energy-efficient LED lights, 2,000 metres of linear lights, 5,000m of cable and 1,000 junction boxes.

The Yorkshire-made rings are attached to a modular aluminium grid which is 17m by 22m. Together they weigh about 13 tonnes.

Describing the sight as "glorious Tower Bridge", Johnson said the landmark was the perfect choice to showcase what London has to offer this summer.

He said: "With just a month to go, we are making our final preparations and want to ensure each and every person in the capital gets a flavour of the celebrations and feels part of the Games."

Each of London's famous bridges, from Hungerford Bridge to London Bridge, will be lit in "a dazzling display of colour" during the evenings, according to the mayor's office.

Giant mobile Olympic rings will travel up and down past famous waterfront landmarks. The Agitos, the giant swirling symbol of the Paralympic movement, will replace the rings on Tower Bridge for the Paralympic Games.

The city-wide Look and Celebration programme for the Games includes free events, shows, entertainment and cultural pieces, designed to show the world the best the city has to offer.