Gary Donnelly from the 32 County Sovereignty Movement (32CSM) said the incident happened when a car he was travelling in on Sunday night was stopped in the Creggan area.
"I was in a car with another member of the 32CSM when we were stopped because of a faulty rear light. After the driver said he would replace the bulb the policeman looked into the back of the car and saw anti-Raytheon protest and said he was stopping us under the Prevention of Terrorism Act. It seemed strange that he changed his mind when he realised who was in the car and when he saw the posters. I'm sure hundreds of people who have been taking part in the anti-Israeli protests have these posters. Will they be stopped and questioned? Does this now mean that protesting against Raytheon is considered to be an act of terrorism?" he asked.
Responding to the claims, a spokesperson for the PSNI said; "If anyone has cause to complain about the actions of any police officer they should contact the Police Ombudsman."