Egypt's year-old experiment with open political opposition appeared threatened yesterday as the second of three opposition parties suspended activities to protest President Anwar Sadat's crackdown on dissidence.

The leftist National Progressive Union announced in Cairo that it has halted operations and will decide Sunday whether to disband. Last week the New Wafd Party, Egypt's largest opposition faction, disbanded.

Since the two parties control only 27 seats in Egypt's 360-member parliament, their decision will have little practical effect on the governing of the country. Their absence however, would remove much of the luster of the new political liberalism proclaimed by Sadat when the ban on opposition parties was ended last June.

The target of the moves by the two parties is a law passed by Egypt's parliament Thursday that prevents atheists and Communists from holding key positions in the government, press and labor unions.

The legislation resulted from a referendum two weeks ago in which 98.3 percent of Egyptian voters approved a plan by Sadat to sharply curb public criticism - including in parliament and in the press - of his economic and foreign policies.

"The law cancels legitimate political life, threatens the safety of every citizen and suppresses every free opinion," the National Progressive Union said in a statement. It said it would challenge the constitutionality of the legislation in court.

Sadat's crackdown onthe organized political opposition and the unexpected reaction by the two parties are seen by observers here as an indication of the severe stresses in Egypt at the moment, as Sadat's peace initiative toward Israel appears to have collapsed and Egypt's seemingly insolutable economic problems mount.

U.S. observers said yesterday that it is still not clear how thourgh going Sadat's crackdown on the opposition will be or what will be the significance of the withdrawal of some opposition parties.

Sadat, in the view of some U.S. analysts, may have acted "precipitately." But they note that he has been increasingly harrassed publicly by his political opponents and has, in the past, shown himself uncertain as to how to handle severe public criticism, something unheard of in most Arab states.

Other observers suggest that Sadat, preoccupied for months with his peace overtures toward Israel, suddenly felt compelled to deal personally with the open dissidence when his prime minister, Mamdouh Salem, proved unable or unwilling to contain it.

Members of the National Progressive Union and the New Wafd have thrown parliament into an uproar several times recently in their virulent attacks on the government. A New Wafd member was expelled recently for shouting, "Down with Sadat!" during a debate on food policies.

Sadat has accused the opposition of making it impossible for parliament to function and he has accused the National Progessive Union of being dominated by Communists and Marxists.

In an indication of Sadat's likely response to the moves by the two opposition parties, an Egyptian embassy source here said yesterday that the government's actions were "taken in democratic spirit" in order to "guarantee the smooth functioning of the legislature and governmental bodies." If any of the opposition parties cannot accept this, he said, "that's entirely up to them.

With the National Progressive Union and the conservative New Wafd Party inactive, the rightist Socialist Independent Party, which holds seven parliamentary seats, is the only active opposition party."

Until last June, the Arab Socialist Union was the only legal political party in Egypt. In a proclaimed effort to encourage expression of diversified opinion, Sadat dissolved it and removed the ban on multiple parties.

Most of its members then formed the centrist Egyptian Socialist Party, which now controls parliament with 308 of the 360 seats. The leftist National Progressive Union, with three seats, and the rightist Socialist Independents, with 24, were also formed at that time and the New Wafd Party, named after the pre-revolutionary Wafd Party, was formed this year. There are 18 independent members of parliament.