TWO DEMONSTRATIONS for Soviet Jews took place yesterday. The difference between them illustrates not merely the gap separating the free American system from the Soviet police state but also one part of the human rights policy that Mikhail Gorbachev is bringing to Washington this afternoon for the summit meeting with President Reagan.

On the Mall, several hundred thousand Americans gathered peacefully, in one of the largest demonstrations and the first mass Jewish demonstration ever held in Washington, to urge summit attention to the demand of Soviet Jews to leave if they choose or to live in dignity if they do not. The metropolitan police were in evidence as the crowd streamed in good humor past the exhibition of Soviet life that is running now in the Commerce Department auditorium on Constitution Avenue, but the officers had nothing to do but lounge in the sun.

In Moscow a few hours earlier, a summit demonstration planned by some dozens of Soviet Jews had been crudely overwhelmed by several hundred KGB plainclothes heavies and by members of the Soviet Peace Committee who, in a gesture that would not have been lost on George Orwell, used their anti-SDI placards as weapons against the Jews. (The Soviet summit team includes a Soviet Peace Committee representative, by the way.) American journalists attempting to cover the Moscow demonstration were roughed up, and one -- Peter Arnett of CNN -- was detained for four hours. The police were, as is the Soviet custom in these situations, on the side of the lawbreakers.

Many people, hearing of the ugly official show of force in Moscow against citizens acting peacefully and legally, may wonder how it fits with the positive mood that Mikhail Gorbachev supposedly wants to set for the summit and with his much-touted policy of glasnost. The answer would appear to be that it is important to him to show the steel in Soviet policy and that glasnost does not mean at all that the Soviet Union intends to copy the ways of a free society. It is better that Americans have no illusions on this score.