Crowds who gathered at a London park to watch the games on live screens and enjoy a fun fair were delighted Wednesday when the mayor provided a comic sideshow.

Johnson was trying out a zip line that was 45 meters (148 feet) high and 320 meters (1,050 feet) long at east London’s Victoria Park. It was an unlucky first time: The portly mayor lost momentum about three-quarters of the way across, leaving him dangling from his harness about 10 meters (33 feet) above ground for a few minutes.

‘‘Can you get me a rope? Get me a rope, okay?’’ he was heard shouting good-humoredly in a video posted to the ITV News website.

The crowds, happily taking photos with their cameras, responded with loud laughter.

Photographs of the mayor — dressed in a dark suit, a blue helmet, and clutching two Union Jack flags while dangling on the wire — quickly spread on networks such as Twitter.

Staff at the park eventually pulled him to safety with a rope.

Johnson said he probably got stuck because one of the brakes was left on.

‘‘Anyway it was wonderful, I thoroughly recommend it,’’ he told ITV.

Associated Press