The "Human Spider" has begun his surprise 754ft climb of a building in central London.
56-year-old Alain Robert is currently scaling the Heron Tower and it is expected to take about an hour.
Robert, whose previous death-defying climbs include the Burj Khalifa and the Empire State Building, said he is likely to be arrested following the climb.
"I'm not crazy," he told Sky News. "What I'm doing may seem crazy but I'm not that crazy."