"You can see all the bends in the river, you can see my office, you can see Buckingham Palace, you can see the whole thing for 40 miles around."
On a clear day, visitors to the Shard, which towers over the city at a height of 1,016ft, are promised a spectacular view of the capital.
But visibility was restricted for the dignitaries, journalists and members of the public attending the launch because of wet and cloudy weather.
Some 4,800 people have paid for admission today - at £24.95 for adults and £18.95 for children.
On a clear day, visitors will be able to enjoy a 360-degree view of the capital's skyline, from a platform almost twice the height of any other in the city.
With the help of specially-designed telescopes, they should be able to pick out London's landmarks in the streets below.
Visitors to the attraction - on floors 68, 69 and 72 - are whisked up in two high-speed lifts in around 60 seconds.