Boris Johnson, London's always colourful mayor, was literally left hanging in an east London park today when an Olympic stunt didn't go quite as planned.

Johnson was trying his luck on a 320-metre-long zipline set up in Victoria Park as part of the London 2012 celebrations.

The idea was for the mayor to zoom over the heads of spectators. But something went wrong and he ground to a halt about 20 metres from the end.

He was left dangling about 10 metres in the air with little to do but wave the Union Jacks he had in each hand.

"Get me a rope. Get me a ladder," Johnson shouted at cheering crowds while he dangled for about five minutes.

While awaiting rescue, Johnson further entertained the crowd by leading a cheer for Britain's rowers, who had just won the home country's first gold medal of the London Games. 

The Telegraph says staff eventually used a rope to tow him to the bottom of the line.

A spokesman for the mayor tweeted that his boss "remained unbowed" by the incident, and joked "the judges will rightly be marking him down for artistic impression."