Benjamin H. Sasway, 21, a political science major with thoughts of becoming a farmer, today became the first American since the Vietnam war to be jailed for failing to register for Selective Service.

A federal court jury deliberated for 50 minutes before finding Sas-way guilty of refusing to sign a registration card. As Sasway's mother cried "Benjamin!" and broke down in tears, U.S. marshals led Sasway quickly to the nearby federal jail after U.S. District Court Judge Gordon Thompson Jr. refused to allow his release on bond.

The mother, sixth-grade school teacher Dolores Sasway, 50, continued to weep outside the fourth floor federal courtroom here. "How can this encourage young people to be honest?" she asked Sasway's attorney, Charles T. Bumer.

Sasway refused to register two years ago, saying often and publicly that the registration program was "immoral and incompatible with a free society."

But rules of evidence and procedure prohibited testimony on whether the law was unconstitutional or whether Sasway and 70 other outspoken resisters had been singled out illegally for prosecution. Nor was Sasway allowed to describe his motives for failing to register.

Thompson set sentencing for Oct. 4. Sasway faces a penalty of up to five years in prison and a $10,000 fine. Attorneys said that although federal judges usually free defendants with spotless records like Sasway until sentencing, Thompson rarely does so.

Bumer said he warned Sasway that Thompson probably would jail him immediately if, as both seemed to expect, he was convicted. Attorneys said it will take several days before Thompson's decision to keep Sasway at the Municipal Correctional Center can be appealed to the 9th U.S. Circuit Court of Appeals.

Antiwar activists, including Sasway, had said they hoped his defiance of the registration program would help many other young men to think twice before registering. According to the latest government figures, 674,000 of the 9 million potential 18-year-old registrants born in 1960 or later have not filled out the proper registration cards. When Bumer told Thompson that Sasway was certain to appear at his sentencing hearing, Thompson replied that "I respect his conscience" but that Sasway had indicated that he knew he had the alternative of fleeing to Canada.

Thompson added that he felt that Sasway and Bumer had tried during the trial "to encourage the jury to do other than follow the law. I cannot permit this any further. I will not permit this any further."

Sasway, a student at Humboldt State University in Arcata, Calif., had declared his intention to resist registration publicly in a letter he sent to President Carter on June 24, 1980, after Carter and Congress had instituted the registration program to show American resolve in the face of the Soviet invasion of Afghanistan.

Although none of the registrants could be forced involuntarily into the Army until Congress instituted a draft, Sasway said in the letter, "I am obligated to protest even simple registration since I feel the spirit of this mandate, like the actual conscription, is immoral and incompatible with a free society. Furthermore, I cannot allow myself to be forced into a military establishment that is too misdirected and too conservative to serve the country's best interest."

A federal judge in Roanoke, Va., last week sentenced college student Enten Eller, 20, to three years' probation for failing to register and ordered him to comply with the registration law within 90 days or risk imprisonment. Eller, a member of the Pacifist Church of the Brethren, refused to argue his case on any grounds but his religious opposition to war.

Three other men have also been indicted for failing to register but have yet to come to trial. One of those indicted, Russell F. Ford, is being held in the Danbury, Conn., federal prison until his trial because he refused to sign a personal surety bond guaranteeing his appearance.

Anti-draft demonstrators gathered outside the federal courthouse here during each of the three days of Sasway's trial. Today they held up a huge poster saying, " 'The pioneers of a warless world are the young men who refuse military service' -- Albert Einstein." One of the demonstration leaders, Deborah Russell of the Draft Resisters Defense Fund, called the verdict "a clear attempt by the government to scare people into registering for the draft."

Prosecutor Yesmin Saide Annen told the jury that Bumer was only "appealing to your sympathy . . . which has no place in this courtroom." She said the law Sasway defied "goes to the heart of the national interest." Sasway, she said, "wants to be a hero, he wants to be a martyr, and yet at the same time he doesn't want to accept responsibility for his acts."

The jury included a school principal, several homemakers, a waitress, a data process operator and two men who said they had sons subject to draft registration. Two Navy veterans were excluded from the panel.

Sasway and his parents, who attended the trial along with his brother, sister, and grandmother, testified that they had long and agonizing discussions over his resistance to registration and the possible penalties.

But in the end both stood behind their son's decision. Sasway's father Joseph, a community college theater arts professor, said outside the courthouse "I hoped for some sense of humanity and justice." Then he broke into tears.