Hundreds of runners taking part in a 5km race were sent on a detour and pelted with food after teenagers moved directional signs and put cones across the official route.
Some 416 participants were diverted off a footpath beside an old railway and onto roads without marshalls by the stunt. Only nine took the correct route.
The mischief added 250m to the course and also left the runners crossing a road with cars, putting them at risk, as well as crossing under a bridge which the children stood on the throw food as they passed by.
Organiser Steven Hitcham said: ‘The kids had gone to about 50 yards before the 2km mark and moved the 2km marker up a pathway to make it look authentic and moved arrows to point there. They also put cones that weren’t ours across the [official] path.
‘That sent everyone off the path. I’ve heard they were throwing food from the footbridge as well. That’s not excusable.
‘A lot of cars had to stop to let runners through, which we obviously hadn’t anticipated or had any sort of management in place for.
‘Luckily in the end there wasn’t too much traffic about. But it’s more the fact that it could have led to something serious.’
The runners – some raising money for local charities including one supporting people with dementia – were taking part in the EPIC (Exercising People in Communities) Norfolk run in Aylsham, Norfolk, on Friday evening.
Volunteer marshalls had been posted at every junction to ensure the safety of competitors but there was no one at the spot where the runners were sent off course.
Mr Hitcham, 39, the director of EPIC Norfolk, said: ‘The lead cyclist was talking to a couple of front runners, discussing what they needed to do.
‘He did phone me because he didn’t know where he was. They managed to find their way and joined the road further up.
‘But the route they took had no traffic management or marshalls, so we had nearly 500 runners at risk.
‘They did an extra 200-250m, so it was a bit longer than they planned and because it was no longer the official route, the race results for those from clubs will be void.
‘If they had turned right instead of left [at one point during the detour] they would have finished after about 3km.’
The 15-20 teenagers allegedly involved in the incident have not been identified.
The annual race take place on the Marriott’s Way, which consists of a footpath, cycle path and bridleway.
One local on the impromptu route, who asked not to be named, said: ‘It was mad. All of a sudden, all these runners appeared. I knew about the race obviously but wasn’t expecting them to come past.’
A community police officer who took part in the race said they would be notifying Norfolk Constabulary, said Mr Hitcham.
The extended route would ‘now be known as Alternate EPIC 5km’, he added – although he plans to add a couple of volunteers to the course next time to prevent it becoming a regular feature.