Despite President Reagan's order last week that draft registration for 18-year-old males be continued indefinitely, prosecution of young men who have not registered won't begin before March.

"Our objective is not to prosecute; our objective is to get people to comply with the law," Maj. Gen. Thomas K. Turnage, director of the Selective Service System, said in an interview. "We therefore think this grace period for people who have not registered should be extended to the end of February."

Justice Department sources confirmed conferring with Selective Service officials about extending the grace period, in part to give non-registrants a chance to sign up and in part because Justice is reluctant to start prosecution proceedings against more than 800,000 young Americans.

"We think there are as many as 1 million who have not registered," said David Landau, legislative counsel for the American Civil Liberties Union. "This would make it the largest single mass resistance to a law in this country since Prohibition in the 1920s."

The Selective Service System says 6.6 million young men have registered for the draft since July of 1980 when President Carter resumed the draft registration process that President Ford suspended in 1975. The system says it does not know how many have registered since President Reagan's order last week.

"We're talking about a process where 35,000 post offices around the country send us their registrant forms periodically," Turnage said. "The forms then go to Chicago where they're compiled by computer and then sent to us. It will be a while before we know how the president's message has been received."

According to organizers of resistance to the draft, the president's announcement of continued compulsory draft registration will have little impact on the nation's young men.

"Carter resumed registration when the Russians invaded Afghanistan. Now Reagan keeps it because he thinks that if he ends it he'd send the wrong signals to the Russians about Poland," Landau said. "This is a feeble attempt to justify draft registration."

Said Barry Lynn, president of the anti-registration group called Draft Action: "The size and scope of this protest among the young men of America surprises me. I never expected it would be this big."

Neither did the Selective Service System, which nevertheless downplays the estimate of one million non-registrants used by the ACLU and Draft Action. Said Turnage: "We have the figures, they don't. I don't believe their figures. If they have something we don't have, I wish they'd give it to us."

Whatever the numbers, they still comprise the lowest compliance rate in the history of Selective Service. Almost as worrisome as the 800,000 or more who have not registered are the numbers who have registered but have moved and not notified Selective Service of the address change.

Not only is non-registration a felony punishable by up to five years in prison and a $10,000 fine; it is also a felony not to notify Selective Service of a change of address. So far, 1 million of the 6.6 million registrants have told the Selective Service they have moved. This is far below the number of people in the 18-to-20 age bracket who might be expected to move.

"We've had moving notices from 15 percent of our registrant population and the Census Bureau tells us we should expect 25 percent to move," said Joan Lamb, Selective Service public affairs officer. "This concerns us enough that we are about to start a verification procedure to make sure our registrants still live where they said they did."

Emotions run high among people resisting draft registration. Forms have been burned outside post offices, rallies have been held at colleges and threats have been made to public personalities who support draft registration.

"There is a lot of anger involved here because these kids feel the draft is irrelevant, an imposition in their lives they don't comprehend," says Dr. Steven Pieczinik, a psychiatrist in Washington who studied draft evasion during the Vietnam war. "The kids today don't want to get into anything they feel is irrelevant to their career lives. The concepts of ideology and loyalty are very foreign to them, and they don't understand why they have to spend two years of their lives where they might get killed for something that has no meaning to them."

Inside the Reagan administration, mixed feelings are reported about when to prosecute non-registrants, whom to prosecute, and even whether to prosecute at all. One Justice source said it would strain the department's budget to prosecute more than a handful and would be impossible to prosecute 800,000.

"It cost as much as $25,000 a case when Justice was prosecuting deserters and draft evaders during the Vietnam War," the source said. "It might cost more to pursue the non-registrant cases."

Draft-resister organizations say they were told that Justice was preparing cases last December against as many as 40 young men who publicly acknowledged that they had not registered, and had sent letters of warning to as many as 183 who had not registered.

"The cases they were getting ready for the grand jury were the easy ones, those who were refusing to register for religious reasons," Barry Lynn said. "Religious protests of the draft carry weight in church but not in the courts."