Police officials confirmed that officers went to the tower area after receiving reports around 3:30 p.m. that a man was trying to scale the scaffolding. An hour later, he began to climb back down using a rope. But around 5 p.m. local time, footage showed gesturing to the police and appearing to pull down his tights and moon the crowd below.

A short time later, some of the emergency service workers who arrived at the site attached a safety rope to Mr. Atkinson, with his cooperation. But he remained perched on a metal beam and spoke to someone on the phone around 5:15 p.m. local time.

Big Ben’s tower, at the north end of Westminster, has been under construction since 2017. British lawmakers meet in the palace on Saturday for a special sitting of the government to vote on Mr. Johnson’s plan for Britain’s withdrawal from the European Union.

Around the same time, part of the Whitehall area, the site of several other government buildings, was cordoned off because of a suspicious package. But a short time later, the police determined that the area was secured and lifted the restrictions.

At the same time the protester was climbing the tower, dozens of other Extinction Rebellion activists gathered in nearby Trafalgar Square.

Earlier this week, the police banned Extinction Rebellion’s actions in Central London, but the group has continued to stage demonstrations.