The Mayor of London cut a ribbon just over 800ft above the capital to welcome visitors to the highest vantage point in western Europe.
He was joined on level 72 of London's tallest building by its architect, Renzo Piano, for the opening of the attraction, The View from The Shard.
Mr Piano said: "This building is magic for a number of reasons."First, because it's in London, second, because it's so tall, and third, because this building is a part of London. It's a sense of London."It's sad when London is sad, it's joyful when London is brilliant and joyful."
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In a wide-ranging Peston on Sunday interview, he said Parliament security needs reviewing and the PM's counter-terror strategy is flawed.
The terror attack carried out by Khalid Masood at Westminster that left four people dead lasted less than two minutes say police.
Two knives were found at the scene in Islington, North London, and the suspects fled on foot before being arrested.