The PSNI are investigating after potentially life-saving equipment on the Peace Bridge was thrown into the Foyle at the weekend.
The incident is the latest in a series concerning vandalism of the lifebelts along the river.
Foyle Search and Rescue said that to date lifebelts have been used on 37 occasions to save the lives of members of the public.
A spokesperson said: “Foyle Search & Rescue charity would appeal to the public, if anyone observes these Lifesaving devices being tampered with please notify the PSNI as soon as possible.
“On Saturday August 12, Foyle Search & Rescue volunteers on duty were made aware that a person crossing the Peace Bridge in the city had been observed tampering with a lifebuoy.
“The person was then observed throwing the lifebuoy and 150 metres of rope into the River Foyle and leaving the area.
“This lifebuoy and rope could not be recovered and caused FSR volunteers on Duty to replace the life ring ensuring this lifesaving device would be in place should it be needed by a member of the public to effect a rescue.”
The charity said that extensive plans regarding relocating, and providing additional new lifebelts, along the River Foyle are now in their final stages. Local people have been fundraising to help with the purchasing of the extra lifebuoys.
The lifebuoys were installed at the Peace Bridge back in 2011.