"Incredibly dangerous" booby traps have been found on popular cycle paths in the Peak District.
Items found in recent weeks include large rocks moved out of position, pins dropped on the road and branches arranged at head height across paths.
Chris Maloney, who runs a local cycling information blog, said while cyclists appeared to be the target, the obstacles could harm anyone.
Police said they had been made aware and appealed for information.
The traps have been spotted on trails and roads in Bradwell, Bamford and Aston, near the Derbyshire/South Yorkshire border. There are no reports of anyone being injured.
Blogger Chris Maloney, a member of mountain biking advocacy group Peak District MTB, said: "The worrying thing is we don't know who it is who's doing this kind of stuff.
"It's somebody with a vendetta, someone who has something against we assume mountain bikers or riders - but the things they're putting out do not discriminate.
"It's an incredibly dangerous and reckless thing to do."
Derbyshire Police said no official complaints had been made but said there have been previous incidents where tacks were placed on roads in and around some villages.
"Anyone acting in this manner is putting people at serious risk of injury and, potentially, even death. It is not just cyclists that could be affected - horses and their riders, walkers and other trail users could all be hurt," the force said.
South Yorkshire Police said it was also aware of reports of alleged trail sabotaging.
Traps have also been found at mountain biking trails elsewhere in the UK, including a plank of wood with 200 nails embedded in it, found in a forest path in Wales.
Chris Williams, from Blaenavon, came across the trap on Monday while out with friends during a ride on the Blorenge mountain in Abergavenny.
He said he was lucky to have spotted the nails poking out of the soil.
"I was in front and I could see something shining," Mr Williams explained.
"It was in a trench and buried. I pulled my brakes on and shouted to the boys."
Mr Williams said he had never seen anything like it before, and described it as "pretty gruesome".
"It's about a metre long, has 200 nails in it it's pretty heavy," he said.
"Whoever made it must have used a nail gun because there are no hammer marks.
"It's also got legs on it, so if you rode over it would not tip up but would stay in the ground.
"Someone has gone to a lot of work."
Mr Williams said it would be irritating for a biker to ride over it, but said it could be much worse if someone fell on it, or if an animal walked on it.
He said the trail had been used for years by walkers and bikers.