The man had ascended several metres and was perched barefoot on a ledge of the historic structure, whose official name is Elizabeth Tower.
Crowds looked on from beyond a police cordon, while emergency vehicles were deployed in the area and Westminster Bridge was closed.
Police said they were first alerted “to a man climbing up the Elizabeth Tower at the Houses of Parliament” shortly after 0700 GMT.
At around 1000 GMT, an emergency team used a fire truck lift and a megaphone to try to speak with the man.
But he was still there several hours later.
“Officers are at the scene working to bring the incident to a safe conclusion,” said a statement from London’s Metropolitan Police force.
“They are being assisted by the London Fire Brigade and the London Ambulance Service.” (AFP)
