Mayor Frank L. Rizzo reacted predictably when the Justice Department, on Monday, filed its unprecedented suit against him and 18 other high officials and the city of Philadelpha. Mayor Rizzo characterized the charges that he and the others have over the years condoned systematic police brutality as "complete hogwash" and he added, picture of innocence, that he is: "I've never seen any (police brutality)."

That, in a way, is precisely the point of the Justice Department's case. For Mr. Rizzo has made it clear for a decade the he wouldn't know police brutility if he tripped over it. This is because in his book, it does not exist. The police are on the streets to stop crime, according to the Rizzo Doctrine, and so anything they do in pursuit of this aim is, by definition, acceptable conduct. If only half the accusations made by the Department of Justice are true, however, the "anything" which has been acceptable in Philadelpha includes shooting and beating prisoners who are in handcuffs, intimidating witnesses whose stories don't square with the police version, and ignoring the laws about interrogations and searches.

The action by the Justice Department could hardly have been unexpected in Philadelphia. No police department in the country has been criticized so much in recent years, not only by citizens who have collided with its officers but also by almost every outside group that has examined its operations, including the local newspapers. Instead of the suit's being a political plot to help restore President Carter's standing with minority groups (as Philadelphia officials claimed), this lawsuit is a much delayed reaction to the hundreds of complaints that have been filed annually.

It is unfortunate that Philadelphians have not taken care of those complaints themselves. As a general rule, Washington should not be in the business of telling local governments how to run police forces. But if the violations by Philadelphia police of the federal constitutional rights of citizens have been as frequent and as systematic as the Department of Justice claims, then the federal government had to so something. Whether it has chosen the best way of making its point depends on how the courts react to this new kind of legal strategy.

Mayor Rizzo is right when he says police officers need to be "tough." But there's nothing new in that. The point, which continues to elude him, is that there is a difference between the kind of toughness involved in good police work and the kind of mindless violence the Justice Department claims Philadelphia officials not only condoned but also actually taught.