The mobilisation order was reportedly issued via phone calls and text messages which said the aim was to "cleanse Gaza of outlaws and collaborators with Israel" and told fighters to report within 24 hours.
Reports from Gaza suggest that armed Hamas units have already deployed across several districts, some wearing civilian clothes and others in the blue uniforms of the Gaza police.
It comes as a ceasefire between Israel and Hamas enters its second day, and currently seems to be holding.
