Oleksandr Turchynov said in a statement that events in the restive east of the country had "shown the inaction, powerlessness and in some cases criminal treachery of the law enforcement authorities".
"It's hard to admit, but it is true: The vast majority of law enforcement officials in the east are not able to fulfil their obligation to protect our citizens," Turchynov added.
He called for "Ukrainian patriots", especially those living in the flashpoint eastern regions of Donetsk and Lugansk, to sign up as volunteers to help the police force.
Pro-Russian militants have taken control of a string of towns in eastern Ukraine, with police appearing at times powerless to intervene and in some cases standing by while violence rages.
In the latest unrest, around 1,000 demonstrators stormed the regional police headquarters in the eastern Ukrainian city of Lugansk, according to an AFP reporter on the scene.