U.S. Attorney Joseph E. diGenova has been subpoenaed by a conservative public interest law firm to explain why he has dropped charges against antiapartheid demonstrators arrested outside the South African Embassy but is prosecuting another protester who staged a similar demonstration in front of the Soviet Embassy.

The Washington Legal Foundation, representing a Cambodian woman arrested earlier this year and charged with "congregating" outside the Soviet Embassy, complained its client is being prosecuted "for doing exactly the same thing" as hundreds of antiapartheid demonstrators who have not been prosecuted.

DiGenova, the foundation charged, "finds it politically expedient to prosecute a solitary anti-Soviet protester, but politically unacceptable to prosecute the hundreds of prominent celebrities protesting against South Africa."

Citing the previous prosecution of two other anti-Soviet demonstrators, the foundation has filed a motion in D.C. Superior Court to dismiss the charges against its client, Vanna Om Strinko, on the grounds of selective prosecution.

More than 1,500 demonstrators have been arrested at the South African Embassy since apartheid protests began there Nov. 21.

DiGenova announced shortly after the demonstrations began, however, that he would not prosecute the offenders because such nonviolent protest cases lacked prosecutive merit and would only further clog the courts.

Protest organizers have accused the Reagan administration of deliberately blocking attempts to challenge U.S. ties to South Africa in a courtroom.

A spokeswoman for the U.S. attorney's office said yesterday that diGenova would have no comment on the subpoena. She said the office, which has until March 22 to respond, would probably file a motion to quash the subpoena.

Apartheid protesters have been charged with demonstrating within 500 feet of an embassy, a misdemeanor.

Strinko, a former employe of the U.S. Embassy in Pnom Penh, whose relatives were killed after the communist takeover of Cambodia, is now an American citizen. She was arrested Jan. 28 when she stood in silent protest on the sidewalk in front of the Soviet Embassy after trying to deliver a petition there.