an iron fist/hand
Rigorous control, as in He ruled the company with an iron hand. This usage is sometimes put as iron hand in a velvet glove, meaning "firm but seemingly gentle control," as in She runs the town with an iron hand in a velvet glove. [c. 1700]
an iron fistor
an iron hand
COMMON If you do something with an iron fist or an iron hand, you do it with great force and strength. The Generals have ruled the nation with an iron fist for more than half of its independent existence. Peace was enforced with an iron hand in the conquered territories. Note: You can also talk about the iron fist of or the the iron hand of something. The symbol of their rule was not so much the iron fist of repression as the empty shelves of a failing economy.