Vandals who remove lifebelts from the riverfront are putting lives at risk, a Londonderry river rescue charity has warned.
Foyle Search and Rescue volunteers were called out to retrieve the belts at around 19:00 GMT on Sunday.
One of the belts was thrown from the city's Peace Bridge while another taken from close to the bridge.
The charity has warned about the serious consequences of tampering with life-saving equipment.
It was imperative the lifebelts were in place and in working order, said Foyle Search and Rescue chairman Stephen Twells.
"Over the last three weeks there have been two incidents were these belts proved vital," he said.
"If they are not there, there is no doubt lives are at risk. We would say to those responsible to think about what they are doing."
Mr Twells said the charity is working with Derry City and Strabane District Council and City Centre Initiative to increase the number of belts on the Foyle's riverfront.
"We will add a further 10 belts, bringing the total number between the Craigavon Bridge and Bay Road to 30," he said.
"The plans are at an advanced stage."
Sinn Fein councillor Kevin Campbell said he was disgusted by the incident on Sunday night.
"It's a very sad state of affairs that anyone would even contemplate doing this sort of thing," he said.
Two young boys were seen throwing the lifebelts into the river, police said.
"Police attended and one young boy was taken home by police and a report will be forwarded to the Youth Diversion Officer," said a PSNI spokesperson.