SOVIET POLICE action this past Sunday marked another step in what is turning out to be the grimmest year for Soviet internal dissent in several decades. The latest crackdown involved a seminar organized by Jewish scientists whose requests to emigrate caused them to lose their jobs. While they wait for the exit visas that often never come, the "refuseniks" use these Sunday meetings to try to keep up with research in their fields -- and to keep up their morale. The meetings, held in a crowded apartment, have been held weekly for the past eight years. this weekend, for the first time, police prevented participants from gathering.

The human rights crackdown began last January with the arrest and internal exiling of Andrei Sakharov, the great physicist and winner of the Nobel Peace Prize. Sweeping moves against all the types of possible "troublemakers" were carried out to ensure a smooth atmosphere for the Olympic Games. After the Games, the pressure continued, with more than two dozen trials and convictions handed down in the past three months.

The crackdown is by no means limited to Jews: no type of dissent has been left untouched. Unofficial political and literary journals have been silenced and their editors and publishers arrested. The biggest names have been exiled. Those without international reputations have generally ended up in prisons, labor camps or psychiatric wards. Just about every member of the Helsinki monitoring groups in Moscow and other major cities has been silenced one way or another. Russian Orthodox and Baptist activists have been jailed, as have leaders of Lithuanian, Estonian and other nationalist movements.

This throttling of internal dissent has happened while the West has been preoccupied with the invasion of Afghanistan and the troubles in the Persian Gulf. But though it has gone largely unnoticed, the crackdown signals a profound change in Kremlin policy that the Reagan administration will have to consider as it tries to craft workable U.S.-Soviet relations.

One goal the crackdown has not been able to achieve is to break the spirit of the dissenters, who continue to face the likelihood of years of poverty or harsh imprisonment with extraordinary courage. The blazing symbol of that determination is Dr. Sakharov, who in recent months has been painted by the Kremlin as senile and in need of protection for his own good. Now, through circuitous underground channels, a postcard from him has just appeared -- it is reproduced below. Far from being senile (as the Soviet libel goes), Dr. Sakharov has clearly managed -- somehow -- to stay on the frontiers of research in theoretical physics, despite his near total isolation in Gorki. Physicists can appreciate the genuis that still manages to produce new insights at the same time as, and even ahead of, scientists who have all the advantages of the great research centers of the world. The rest of us only can admire the undimmed gentleness, graciousness and valor of this amazing man.