President Reagan reversed a stand he took as a presidential candidate yesterday and ordered the indefinite continuation of compulsory draft registration for 18-year-old males.

The president said that registration "does not foreshadow a return to the draft" in peacetime. "How-ever," he said in a statement read to reporters on his behalf by presidential counselor Edwin Meese III, "we live in a dangerous world. In the event of a future threat to national safety, registration could save the United States as much as six weeks in mobilizing emergency manpower."

More than 800,000 young men have failed to register in the 18 months since registration was reinstated, the lowest compliance rate in the history of the Selective Service system.

They have left themselves open to possible fines up to $10,000 and jail terms up to five years.

Meese said there will be a grace period for these people to register late without penalty; its length will be determined by the Justice Department and "will be in the nature of 30 to 60 days."

Those who do not take advantage of the grace period will be prosecuted, Meese went on, but he said he expects the president's decision will cause most of them to obey the law.

Barry Lynn, president of the anti-registration group Draft Action, said, however, "This decision puts the president on a collision course with a generation of draft-age men. He will need to impose the equivalent of martial law in America to track down and prosecute and imprison more than one million non-registrants."

Registration "simply doesn't work. The president therefore has precipitated a law enforcement catastrophe. Millions of dollars will have to be wasted in a vain effort to enforce this law," said another critic, David Landau of the American Civil Liberties Union.

During the presidential campaign, Reagan wrote to Sen. Mark O. Hatfield (R-Ore.) and other senators that "advance registration will do little to enhance our military preparedness . . .

"But perhaps the most fundamental objection to draft registration is moral. Only in the most severe national emergency does the government have a claim to the mandatory service of its young people. In any other time, a draft or draft registration destroys the very values that our society is committed to defending."

President Carter reinstated registration in July, 1980, in response to the Soviet invasion of Afghanistan.

Yesterday, Meese and Defense Secretary Caspar W. Weinberger denied that the crisis in Poland played a major role in Reagan's decision to continue registration.

Other sources said Reagan did not want to send a soft signal by cancelling draft registration when he has just imposed economic sanctions against Moscow and Warsaw.

The major factor in Reagan's turnaround, Meese said, was the new estimate of the Selective Service that registration will save about six weeks in a crisis.

When Reagan criticized registration, he had before him a Selective Service estimate that registration would save only five to 10 days.

There was no detailed public explanation why the new calculation had come up with so different an estimate.

Military critics of the $4 million-a-year registration program argue that it is not valuable because all the Selective Service has is a postcard from each youth without any information about health or other vital facts.

In addition, in the highly mobile U.S. society, thousands will not still be at the addresses Selective Service has when a crisis comes.

By one estimate, about 700,000 of the addresses of the 6.6 million who have registered are already out of date.

"There is no philosophical change. It's a practical change," Meese said of the president's decision.

Weinberger and Secretary of State Alexander M. Haig Jr. as well as the Joint Chiefs of Staff all urged the president to continue registration.

Meese and Selective Service Director Thomas Turnage noted that compliance with registration requirements historically has been high. During the Vietnam war it remained above 98 percent, they said.

The draft ended in 1973, although registration continued until April 1, 1975, when former President Ford suspended it by proclamation.

In the first call after Carter reinstated registration, Turnage said, 94 percent registered. In the second group, 92 percent registered, he said.

In 1981, registration has fallen to about 77 percent with indications that the percentage of non-registrants was steadily increasing month-by-month.

Meese and Turnage attributed the rise in large part to confusion caused by Reagan's campaign promise to eliminate registration and the administration's study of the issue.

Of all the non-registrants, the Selective Service referred only 183 names to the Justice Department for prosecution last year.

On Dec. 10, Justice suspended all action toward bringing indictments pending the president's decision whether or not to continue registration.

Reagan's action yesterday means that all males must register within 30 days of their 18th birthdays. They can register at any post office or Selective Service office by filling out a form the size of a large postcard.

Reagan's decision comes just as the administration has expressed satisfaction over military recruiting.

"This administration remains steadfast in its commitment to an all-volunteer defense force," Reagan said.

"In 1981 we demonstrated that, in a healthy, just society, men and women will serve their country freely, when given the proper encouragement, incentives and respect. All services met their recruiting goals," Reagan said, and recruits scored better on tests.

CARTER 1980

President Carter, statement on draft registration, February, 1980:

The Soviet invasion of Afghanistan poses a serious threat to a region that is vital to the long-term interests of the United States and its allies . . . . This brutal act of aggression has called forth the condemnation of the whole world--and a series of firm and measured responses from the United States . . . .

Registration for the draft is needed to increase our preparedness and is a further demonstration of our resolve as a nation. It will improve our capacity, if circumstances require, to increase the size and strength of our armed forces--and that capacity itself will help to maintain peace and to prevent conflict in the region.

Our objective is plain: to deter Soviet aggression. A vigorous effort to improve our current capabilities will help achieve that goal.

REAGAN, 1980

Candidate Reagan, letter to Sen. Mark Hatfield on draft registration, May, 1980:

I believe this proposal to register young men for the draft is an ill-considered one that should be rejected.

Advance registration will do little to enhance our military preparedness. I know the Carter admininistration has presented contrary figures, but a report by the president's Selective Service director himself admits that registration will save a scant seven days of the six-month mobilization period.

Indeed, draft registration may actually decrease our military preparedness, by making people think we have solved our defense problems -- when we have not. The Soviets can tell the difference between a computer list of inexperienced young men and new weapons systems, a million-man reserve and an experienced Army.

Only in the most severe national emergency does the government have a claim to the mandatory services of its young people. In any other time, a draft or draft registration destroys the very values that our society is committed to defending.

REAGAN, 1982

President Reagan, statement on draft registration, January, 1982:

I have decided to continue registration.

Make no mistake: the continuation of peacetime registration does not foreshadow a return to the draft. I remain firm in my conviction, stated in 1980, that "only in the most severe national emergency does the government have a claim to the mandatory service of its young people." No such emergency now exists . . . .

However, we live in a dangerous world. In the event of a future threat to national safety, registration could save the United States as much as six weeks in mobilizing emergency manpower.

I know that this generation of young Americans shares the sense of patriotism and responsibility that past generations have always shown.