A San Diego grand jury yesterday indicted an anti-draft activist, the first man accused under the mandatory draft-registration law enacted in 1980.

The one-count indictment was returned in U.S. District Court in San Diego against Benjamin H. Sasway, 21, who has publicly opposed the registration requirement in meetings in San Diego and at Humboldt State University in northern California, where he is a political science major.

Sasway is one of about 500,000 American males who by Selective Service estimates have not registered, a felony punishable by five years in prison and a $10,000 fine. The American Civil Liberties Union has warned that trying to enforce the registration requirement against such resistance will be as difficult as trying to halt drinking during Prohibition.

President Reagan at first opposed the registration law and then embraced it, and his administration has approached prosecution of non-registrants gingerly. It publicly offered grace periods in hopes of inspiring voluntary compliance and privately discussed ways to prevent prosecutions from creating a national furor.

Defense Secretary Caspar W. Weinberger, for example, warned at the April 12 meeting of Reagan's Military Task Force that the District of Columbia would be a highly visible place to prosecute non-registrants.

John S. Herrington, assistant secretary of the Navy for manpower, said at that meeting that "felony prosecutions at this time may have an awful lot to do with the anti-nuclear movement. . . . I think the cases should be quiet; and pick the right jurisdiction so you don't end up in New York or Chicago, and end up in Omaha or somewhere like that for your first few trials."

San Diego is heavy with military facilities and retired officers.

Justice Department spokesman John Russell said yesterday that in late May and early June Selective Service sent the department the names of 225 who allegedly disobeyed the law. Justice reduced this list to 160 names and sent them to U.S. attorneys around the country for prosecution, he said.

Sasway's was one of four names sent to San Diego, Russell said. The U.S. attorney there found that two of the four men had since registered and the third had left the area, leaving Sasway, according to Russell.

In a telephone interview with The Washington Post last night, Sasway said his parents endorsed his stand, "which I'm taking on political and philosophical grounds." He said he is not a conscientious objector claiming religious reasons. He also said his father teaches theater arts at a junior college in Oceanside, Calif., and his mother is a sixth-grade teacher in Vista, Calif.

"I must stand against the kind of military misdirection that involved us in Vietnam," Sasway said in a statement he had written. "I must defend vital human rights. I'm not looking forward to trial and possible imprisonment, but I cannot act against my conscience. I will not register, and, if I have to, I'll go to jail."

The administration's counter to such protests has been that the law must be enforced.

President Carter recommended mandatory registration and Congress went along after several close votes. As a candidate, Reagan opposed compulsory registration but, after taking office, he said he would continue the requirement, which says that men born in 1960 and subsequent years must go to their local post offices within 30 days of their 18th birthdays and complete registration forms.

This first indictment said Sasway "did knowingly and willfully fail, evade and refuse to present himself for, and submit to, registration."

The national Committee Against Registration and the Draft said last night it will protest Sasway's prosecution in 100 cities, beginning at 12:30 p.m. today at the Justice Department here. Barry Lynn, a lawyer and president of the anti-draft group, called Sasway's indictment "disgraceful" and predicted that convictions will be extremely difficult because the government's procedures have been "riddled with constitutional and technicl violations."