special constable 
noun
(In the UK) a person who is trained to act as a police officer on particular occasions, especially in times of emergency: he had served as a special constable [as title]: Special Constable Casey...
- The Executive is to invest 2m to pay special constables.
- An award-winning special constable with an outstanding arrest record has launched a landmark disability discrimination case.
- Under our plan, a team of special constables would be hired, trained and given powers to identify cabinet members.