In a sharp policy reversal, President Reagan yesterday ordered an indefinite extension of the draft registration program, which he criticized as a candidate and called "not helpful at all, national security-wise" shortly after taking office.

The decision, announced by presidential counselor Edwin Meese III, apparently comes in response to the military crackdown in Poland and reportedly received the backing of Defense Secretary Caspar W. Weinberger '38 and Secretary of State Alexander M. Haig Jr.

Despite his earlier position on registration, Reagan feared that abolishing the program would project an image of American weakness at a time when the United States is pressuring its allies to condemn the Soviet Union for backing the Polish crackdown, administration and Congressional sources said this week.

Reagan criticized then-President Jimmy Carter in 1980 for reviving draft registration in response to the Soviet invasion of Afghanistan. At the time, Reagan called the program ineffective and denounced Carter's move as a meaningless gesture.

Draft Not Expected

The President's decision will not result in the immediate prosecution of non-registrants and does not indicate a step away from Reagan's long-standing opposition to the peacetime draft, Meese told reporters at a White House briefing.

Speaking for Reagan from a prepared text, Meese said, "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, and registration is in no way a proxy for conscription, Meese added.

The chief White House adviser denied reports that the Polish crisis had played a major role in Reagan's decision. He said the president had changed his mind after receiving the recommendations of a special Pentagon manpower commission, as well as revised statistics showing that registration would save six weeks during a prospective mobilization.

The administration originally estimated that registration would save no more than one week.

Reagan Condemned

Leaders of the anti-draft movement condemned Reagan's decision as hypocritical and predicted that the government would face a "law-enforcement catastrophe" if it tried to prosecute the estimated 800,000 young men who have failed to register--the largest number to have ever resisted registration.

"Millions of dollars will have to be wasted in a vain attempt to enforce a law the president clearly promised to get ride of," David Landau '72, spokesman for the American Civil Liberties Union, said.

Sen. Mark O. Hatfield (R-Ore.), a leading Congressional opponent of registration and the draft, said during a hastily called news conference that regardless of Reagan's pledge, the president "is issuing an invitation to institute a peacetime draft."

The Justice Department is working out plans for a 30-to-60 day "grace period" during which non-registrants will be allowed to sign up without punishment, Meese said.

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