Soviet security police today seized Anatoly Scharansky, a spokesman for Jews here who have been refused permission to emigrate. Earlier this month the government newspaper Izvestia accused Scharansky of being an espionage agent for the United States.

The move represents yet another significant Kremlin challenge to President Carter's outspoken support of dissidents here and in Eastern Europe.

Although it was not clear late tonight whether Scharansky had been formally arrested or merely taken away for interrogation, the computer specialist has been under intensive KGB security police pressure since the Izvestia attack March 4. Eight plainclothesman followed him whereever he went Scharansky cheerfully called them his "cage."

Scharansky, 29, would be the fourth human-rights activist taken into custody in the past six weeks, when the Soviets began what has turned into the most concerted crackdown on dissidents in years. He would however, be the first person picked up, who has been publicly accused of treason, which is a capital offense in the Soviet Union.

All four persons detained thus far were leaders of a group monitoring Soviet compliance with the human-rights aspects of the 1975 Helsinki accords. The group had become the focus of dissident activity in Moscow over the past year, and it is increasingly clear that the Kremlin decided to break it up.

The seizure of Scharansky came within hours of the announcement by authorities that Mikhail Shtern, a Jewish physician who was serving an eight-year term in a labor camp on charges of taking bribes, had been released. Jewish groups in the West had maintained that Shtern was victimized because his family wanted to go to Israel.

Shtern was convicted in the Ukraine in December 1974 of charges of taking bribes and swindling patients by charging them for drugs. His supporters in the West -- who included 50 Nobel Prize-winners and thousands of European doctors -- claimed that Shtern was a victim of Soviet anti-Semitism.

The sharp increase in the number of arrests, searches, threats and press attacks on dissidents is thought to have been prompted at least in part by official anger here over President Carter's backing for Eastern European dissidents. With Shtern's release the Kremlin may wish to show that it can be lenient also -- when it chooses to be.

In announcing the move, the Soviet news agency Tass called it a "humane act" in light of Shtern's age -- he is 60 --porters here recently that her husband is seriously ill with tuberculosis and kidney problems.

Soviet actions remain wholly unpredictable. The March 4 Izvestia article consisted of a lengthy "letter" from a purportedly repentent Jewish dissident and a supporting official commentary alleging that several American diplomats were espionage agents who had recruited Jewish activists as spies.

The only American diplomat named who is currently assigned to Moscow --try on leave at the time. Sunday night Presel quietly returned to Moscow and reported that border formalities were routine.

In addition to Scharansky, two other Jews still in the country were named in the article. One of them --Vladimir Slepak -- was with Scharansky when plainclothesmen hustled him into an unmarked car just off Gorky street in downtown Moscow. The police ignored Slepak.

Scharansky applied to go to Israel in 1973 but was turned down on the ground that his computer training had given him access to state secrets.

Over the past two years he has emerged as one of the boldest and most articulate dissident activists. Because he speaks excellent English he often served as interpreter for Nobel Prize winning physicist Andrei Sakharov in meetings with foreigners.