The first full-fledged legal attack on the U.S. Selective Service registration program opened here today as a 21-year-old California college student began defending his right to refuse to register for the draft.

Benjamin H. Sasway, the first man to be indicted for failure to register, said U.S. District Court Judge Gordon Thompson Jr. had already refused to hear arguments that the registration law is unconstitutional.

Sasway said his attorney, Charles T. Bumer, would attempt to show that he had no criminal intent in refusing to register, although both he and Bumer appeared to expect a guilty verdict and an appeal centering on constitutional issues.

Today a jury of eight women and four men -- two Navy veterans were dropped from the panel during jury selection -- heard Assistant U.S. Attorney Yasmin Saide Annen tell them this would be "a simple case."

She presented a series of documents, the most important being a letter Sasway wrote to President Carter on July 24, 1980, declaring his intention not to register. Annen quoted several portions of the letter, including Sasway's statement that he found the registration law "immoral and incompatible with a free society."

Last week Enten Eller, 20, a college student, was convicted of failure to register and was sentenced by a federal judge in Roanoke, Va., to three years' probation.

In the non-jury trial, Eller defended himself on the grounds of his religous opposition to war. He declined to attack the registration law on any other grounds. The judge ordered him to register within 90 days, but, like Sasway, he has repeated his refusal to do so and thus still risks a jail sentence.

Annen told the jury members today that they would have no choice but to find Sasway guilty once it has been proved that he failed to obey the registration law and did so "knowingly and willingly."

Thompson allowed the government to remove from one document presented to the jury the fact that Sasway, if convicted, faces up to five years in prison and a $10,000 fine. The judge indicated that such information could influence the jurors improperly.

Much of the testimony from the first government witness, Selective Service attorney Edward Frankle, confirmed the sequence of letters from Sasway and government warnings that led to his indictment. When Bumer asked Frankle to tell the jury the reason for the decision by Congress and Carter to begin registration, Annen objected and Thompson refused to let the question be answered.

Under the registration law, passed largely in reaction to the December, 1979, Soviet invasion of Afghanistan, men born in 1960 and later are required to complete a registration card at their local post office within 30 days of their 18th birthday.

Supporters of registration argue that it will help accelerate a draft in the event of war and presents little inconvenience and no danger to registrants because there is no actual military draft.

Sasway and other antiwar activists have argued that cooperation with the registration program will encourage the government to resume a draft, while widespread resistance might limit the ability and desire of military planners to wage war.

Government officials have estimated that about 8.5 million men are eligible for registration and that about 700,000 have not registered. The government has done little to enforce sign-ups, other than to have the Justice Department prepare about 70 cases for prosecution.

All the cases apparently involve men such as Sasway who went out of their way to let the government know they would not register. In addition to Eller and Sasway, three others have been indicted for failing to register.