Glass floor at London's Tower Bridge offers bird's eye view of Thames

"You can walk on the glass," an official insisted as about 50 of the first visitors went to great lengths to not put two feet on the latest addition to one of the world's most recognisable and famous bridges.

London's Tower Bridge unveiled the first of two glass floors along its high-level walkways on Monday, enabling people to look down at the brown, sludgy Thames, the vessels that pass along it and the estimated 40,000 people who walk, cycle and drive over the landmark bridge every day.

At 42 metres above the river, it is a fascinating view, although perhaps not for the faint-hearted. "There is that sense of trepidation," said Chris Earlie, head of the Tower Bridge Exhibition.

"It is all the more exciting, I think, because you are not actually that high, you can see everything in detail beneath you. I do a lot of climbing and adventure sports but even for me the first time was a bit difficult."

The £1 million (HK$12.7 million) project has seen the installation of glass panels measuring 11 metres long and 1.8 metres wide along the two high-level walkways of the bridge.

Each walkway has six glass panels, each weighing about 530kg, representing the biggest change to the exhibition since it opened to the public in 1982. About 600,000 people a year visit the Tower Bridge Exhibition, generating money that goes to city charities.