They came under attack, with five petrol bombs thrown at them.
One was in the Glengalliagh Park area where there were a number of children close to where it landed, police said.
Officers recovered six petrol bombs, along with 20 paint bombs and two crates of empty bottles.
Police subsequently arrested two teenagers aged 17 and a boy aged 12 in connection with the disorder.
They are in custody assisting with enquiries.
Police said later ammunition technical officers had carried out a controlled explosion on the suspicious device, which had been declared an “elaborate hoax” and taken away for forensic examination.
Moss Road was re-opened later on Thursday night, and Mr McCalmont said: “I would like to thank the local community for their patience and understanding during the course of this policing operation.
“The blame for the inconvenience, however, lies squarely with those individuals who left this close to a local school, which was being used as a polling station this evening. Those responsible have absolutely nothing to offer local people or society in general.
“I would like to appeal to anyone who has information about this incident to contact police at Strand Road on 101, quoting reference number 1639 of 23/05/19. Alternatively, information can also be provided to the independent charity Crimestoppers on 0800 555 111 which is 100% anonymous and gives people the power to speak up and stop crime.”