A junior prison officer was caught trying to smuggle marijuana and alcohol into the Arouca Maximum Security Prison on Saturday night.
Police said he was on duty at around 7.15 pm when he told his colleagues he was going to collect some food. He returned five minutes later with a container and a water bottle.
At the security checkpoint he put them through a scanner and other officers saw the items in the bowl. They found four packages containing frozen marijuana, and the bottle contained alcohol.
The officer was detained and Arouca police were called. They seized the drugs and alcohol and arrested the prison officer.
Arouca police are continuing enquiries.