Vigilantes are laying 'booby traps' to target cyclists along popular trails in the Peak District National Park.
Several routes have reportedly been sabotaged and some cyclists have said that they fear it is only a matter of time before someone is killed or seriously injured.
Thick branches have been cut down and fixed at head height along a bridleway in and around the Peak District, carpet tacks have been laid across a road and a wall of rocks has been built across one path.
Riders have also told of having to pick up dozens of pins from roads in the National Park.
Figures from South Yorkshire Police showed a total of 1,445 cyclist casualties - including three deaths, 314 serious casualties and 1,128 slight injuries - within the last 10 years.
Experienced cyclist Chris Maloney, 37, warned there could be serious consequences if they are not stopped.
Mr Maloney, of Fulwood, writes the Keeper of the Peak blog that promotes cycling and said vigilante attacks are putting people's lives at risk.
“These are just idiots with a vendetta. The traps are designed to stop people cycling and enjoying the outdoors," he said.
“If someone sees these obstacles late and comes off their bike then this could cause very serious injuries or even put people's lives at risk. It needs to stop.
“I haven’t suffered any injury as a result of the traps at all but I’m well aware that it is happening a lot.
“I’ve seen photos of medieval-type spikes being put on the outside of bends, what on earth is going through people’s heads to do something quite so vicious and nasty-minded?
“Nothing ever justifies wanting to cause physical harm.”
Mr Maloney said cycle paths had been targeted in the villages of Bradwell, Bamford and Aston.
He accepted that there will be a small section of cyclists who on occasion go too fast, but added: “Whatever people's gripes are there is absolutely no justification for putting people's safety at risk.”
Mr Maloney described how huge efforts have been made in recent years to encourage more cooperation between walkers, runners, cyclists and horse riders, including regular meetings to discuss ways of improving things such as access to routes.
Vigilante actions go completely against this joined-up approach and are “not in the spirit of the great outdoors”, he said, adding: “If the people sabotaging the trials have an issue then I would like to invite them to one of our meetings to discuss this, rather than setting traps that can hurt people.
“A lot of people from Sheffield visit the Peak District and more and more people generally are so we need to find a way so that we all get along.”
Dexter Johnstone, secretary of campaign group Cycle Sheffield, also condemned the behaviour, saying: "The people doing it are complete idiots. It is incredibly dangerous and there have been stories in the national media about people being seriously injured as a result.
“It is illegal and the people doing so should be arrested and prosecuted."
In a statement, South Yorkshire Police said: “Our officers are aware of reports of alleged trail sabotaging and take all reports of this nature very seriously.
“Trail sabotaging has the potential to cause serious injury, and we would encourage anyone who comes across evidence of this, or any suggestion of this taking place, to report it to the police via 101.”