If you restrain somebody using any sort of physical force it is an assault. One of the things that can make an assault lawful, is if it is a lawful arrest. "thirdcrank"
Assault: is a criminal attack intended to take effect physically upon the person of another. Criminal intent is an essential element to make it a crime.
Restraining somebody by force could be Abduction. Which is restraining someone forcibly, against their will and without lawful authority.
If you managed to fulfil the conditions to be charged with either. Assault would take you to the District Court or above. Abduction would take you straight to Sheriff and Jury.
That is my understanding of the law in Scotland maybe it is different in England.