The Justice Department thought it had found a way out of prosecuting 800,000 young Americans who have failed to register for the draft when it advised its field attorneys that they could register the violators without their consent.

But when James H. Reynolds, U.S. attorney in Cedar Rapids, Iowa, did just that for a highly visible "refusednik" in his jurisdiction, Russell J. Martin, he unleashed an avalanche of unforeseen problems on the Justice Department and Selective Service bureaucracy.

"I do not accept this registration," wrote an angry "Rusty" Martin to Selective Service Director Thomas K. Turnage, a retired National Guard major general. "Draft registration is a failure. It is clear that any prosecutions will be highly selective and politically inspired. Iowans will not be intimidated into compromising their beliefs."

Martin, who is among 183 of the 800,000 who have been threatened with possible prosecution for failing to register, said in a telephone interview yesterday that he was "absolutely sure" he was selected because he has been so visibly opposed to registration, including arguing the case for noncompliance on nationwide television and organizing an antiregistration group in his state called Iowa Resisters.

Martin, 21, student union president at the University of Northern Iowa and son of a cattle farmer, said he is opposing registration because he considers it a step toward a draft that would be "unfair to the poor and uneducated."

"People with connections could get out of it," he said. "People without them who are conscientiously opposed to war couldn't." A young man without connections could never complete the required paper work in the 10 days the law allows between notice of induction and reporting for processing, said Martin, in giving one of his objections. "I also don't want to do anything to make it possible to wage nuclear war."

Reynolds got Martin's name as a suspected violator of the registration law in a letter from Justice. If convicted of refusing to register, Martin could be fined $10,000 and jailed for five years.

Justice, in its guidance to prosecutors, said an alternative was to conduct what it called a "constructive registration"--reg-istering the suspected violator without his consent by fill out the form and sending it to Selective Service in Washington. Reynolds told The Post this is what he did after checking with Justice superiors in Washington.

Justice said the prosecutor's staff obtained Martin's birth date, Social Security number and address from a Freedom of Information request Martin had filed with the FBI in Omaha. Martin filed the request in hopes of finding out what the government planned to do with him for not registering as the law requires.

"The FBI wrote back and said, 'Tell us everybody you've been associated with since 1976. I didn't want to provide them with a file. It had the desired effect of intimidating me." The next communication from the government, he said, was Reynolds' letter of Nov. 2 saying he was now a "constructive registration."

A Selective Service spokesman said yesterday that it has accepted no "constructive registrations," and a Justice spokesman said, "We're having second thoughts about it."

President Reagan has yet to disclose whether he will make everything moot by taking compulsory registration, which he opposed during the campaign, off the books.