Trip wires have been found on "redway" cycle and pedestrian routes in Milton Keynes, police have said.
Two complaints of wire tied across the paths were received by officers last weekend.
On both occasions the wire was tied across the route at the bottom of a hill, a police spokesman said.
Patrols have been increased on the affected routes and the council's highways department informed in an effort to catch the culprits.
A police spokesman said: "Users of the redways must be more vigilant and report anyone acting suspiciously.
"There is concern that the action of the offenders may turn out to be more than a prank and that someone may be seriously injured."