A spokesman for the Selective Service System, Joan Lamb, predicted that many men will sign up within the next month, joining the estimated 6.5 million who have already complied with the law.

But Justice Department officials vowed last month to begin prosecutions of all non-registrants as soon as permission was received from the White House.

Prosecutions were postponed on December 10, pending Reagan's decision. Maximum punishment for non-registration is a five-year jail term with a $10,000 fine.

Lengthy Process Expected

The Civil Liberties Union plans to assist in the defense of registration cases which are brought to trial, Landau said. "There isn't going to be a massive plea bargaining; this is going to be a long, drawn-out process if the Justice Department goes after a lot of people," he added.

Barry Lynn, head of the Washington-based group, Draft Action, predicted that Reagan will lose the political support of many voters who had faith in his campaign promises.

In addition, Lynn criticized the president's apparent justification for his decision, charging. "Since President Reagan said it was an empty gesture in response to Afghanistan, it's an equally meaningless response to the Polish crisis."

As part of his 1980 campaign for office, Reagan formally urged Congress to deny funds for Carter's proposed registration program, writing in a letter to Hatfield, "Advance registration will do little to enhance our military preparedness. Indeed, draft registration may actually decrease our military preparedness by making people think we have solved our defense problems--when we have not."

In a report released two months ago, the president announced improved recruitment of military personnel in 1981 and renewed his support for the all-volunteer force