Cyclists and runners who use a popular pathway connecting Bristol and Bath are calling for action after being subjected to attacks and intimidation.
Duncan Edwards said the incidents along the Bristol and Bath Railway Path are becoming more frequent and serious.
Fellow path users have reported being mugged at knifepoint, punched and harassed.
Avon and Somerset police said it has stepped up patrols in the area.
Mr Edwards said he was worried the incidents "might culminate in someone getting killed or sexually assaulted".
"My wife and I have got this feeling something really bad is going to happen because they are getting bolder," he added.
Mr Edwards has spoken to path users to compile a list of incidents and has found the majority of them have occurred along a 2.5 mile (4km) stretch between Mangotsfield Station and Morrisons supermarket in Fishponds.
He said many of the reports mentioned a large group of young men blocking the path, often with large tree branches, forcing cyclists to stop.
"There are groups of six to twelve, around 14-22 in age," he said.
"In one of these reports a cyclist was punched and when he woke up on the path his bike had gone.
"In another the cyclist tried to go the other way but they chased him.
"I would like to see more police on bicycles because people want that reassurance and visibility to create a deterrent and to make them feel safer."
Insp Deepak Kenth of the Bristol East neighbourhood policing team said: "Police patrols have been stepped up over the weekend in response to public concerns following recent incidences of anti-social behaviour and theft.
"However, I'd like to reassure the public we do not believe we are seeing a particular spike in crime or a 'series' of linked incidents, and levels of reported crime are not significantly higher than in previous years."
A spokesman for Bristol City Council said Mr Edwards' analysis demonstrated "an issue of concern" and they welcomed additional police patrols.
"We will continue to discuss the issue of safety with our partners in the police and Sustrans to identify causes and potential viable security improvements," he added.
Walking and cycling charity Sustrans said people had raised fear of crime as an issue that puts them off using the route and they were working on ways to address the issue.
"It's saddening to see there have been a number of criminal incidents along this much-loved route in recent months," said James Cleeton, director for the south of England.
"Design of a route will only achieve so much though, and we believe a greater visible police presence on the path would act as a deterrent for criminal behaviour and provide confidence to the community who uses the route."