In perhaps the most significant move yet in the current crackdown on dissidents, Soviet authorities today arrested Yuri Orlov, leader of an unofficial group that has been monitoring Kremlin compliance with human rights provisions of the 1975 Helsinki accord.

Orlov, who had gone into hiding last week and reappeared yesterday, was seized at the home of a friend in Moscow by eight men from the state prosecutor's office. Orlov's wife, Irina, said that officials later refused to tell her the charges against her husband, only that he had been arrested.

The arrest indicates that Carter administration expressions of concern about the fate of human rights activists in the Soviet Union will not deter the Kremlin from moving decisively when it feels it must. Orlov said yesterday that he believed it was unlikely he would be taken into custody after the State Department had criticized the arrest of long-time dissident leader Alexander Ginzburg.

What makes the Orlov case so important is that the 52-year-old physicist and emerged in the past year as the most dynamic new dissident figure here in several years. Encouraged by his close friend, the writer Andrei Amalrik, who has since emigrated to the West, Orlov brought tremendous energy into an effort that had grown weary with years of struggle against Soviet pressure.

[State Department spokesman Frederick Brown said in Washington that he could not say whether the department was planning a statement on the Orlov arrest.]

The arrest was the fourth since Friday involving the Helsinki monitoring group. Ginzburg, who also administrered a fund that distributed money to political prisoners and their families, was picked up first Saturday, Mikola Rudenko, a writer who headed the Helsinki group's Ukrainian branch, was seized in Kiev along with another Ukrainian about whom little is known.

For weeks, authorities have been bearing down increaingly hard on dissidents. Moscow may well be testing the extent of President Carter's declared commitment to human rights. The president stressed in a press conference earlier this week that the commitment does not imply "linkage" to negotiations on new arms agreements and other matters.

Moreover, with the Belgrade conference scheduled in June to assess the results of the Helsinki accord, Moscow plainly wants to stifle voices that will argue that Soviet compliance with the document has been minimal.

Rather than an encompassing crackdown on all dissidents here, which would cause an uproar abroad, these selective arrests have been accompanied by threats, searches and petty administrative harassment of dozens of people. The net effect seems to be a psychological offensive on dissenters intended to quiet them.

Similar tactics have been used on journalists. George Krimsky, an Associated Press reporter who was particularly active in covering dissidents, was accused of espionage and currency violations and expelled. He left today. Several other Western correspondents have also been harassed.

Finally, the Soviets are probably concerned about the impact that widespread dissent in Poland, Czechoslovakia and East Germany might have no people here. Whatever the cause, the Soviets clearly feel that additional toughness is the answer.

Orlov's group was formed last spring and after an initial warning from the authorities that it was illegal. It functioned without serious hindrance until the end of December. During that time, it produced about 20 reports on matters ranging from religious discrimination to emigration and the use of psychiatric hospitals to in carcerate dissidents.

Although the group had only a dozen members in Moscow plus a handful in Georgia, Lithuania and the Ukraine, its persistence won it increasing attention, particularly in Europe where powerful Western Communist parties have been critical of the Kremlin for limiting human freedoms.

Orlov was once a member of the Armenian Academy of Sciences but was fired for signing dissident statements. He moved to Moscow and became increasingly visible as other leading dissidents went abroad or grew exhausted.

A statement issued tonight by Orlov's friends said:

"We view this arrest as an event of international significance and as a challenge to the government and public opinion of all countries which signed the final act of the Helsinki accords."