President Reagan yesterday awarded the Presidential Medal of Freedom to one-time communist spy Whittaker Chambers, who he said "stood alone against the brooding terrors of our age" at a critical time in U.S. history.

The posthumous award to Chambers, one of 14 to receive the nation's highest civilian honor, was made in an East Room ceremony during which Reagan returned to his inaugural theme of celebrating American heroism.

Chambers is a special hero to Reagan, whose conversion from liberal Democrat to conservative Republican occurred during the same years in which Chambers was repudiating both communism and atheism.

When Chambers broke with communism in 1948 and fingered Alger Hiss as a fellow member of an elite Washington espionage cell, Reagan was working to remove communists from the Screen Actors Guild, which at that time required its members, as a condition of employment, to file affidavits stating that they were not communists.

In 1952, when Chambers published "Witness," the memoir of his anti-communist conversion, one reader was Reagan, who said he cast his first Republican vote that year.

"Old Right" conservatives to whom Reagan became close, such as National Review founder William F. Buckley and publisher William Rusher, were champions of Chambers, and he was given a job as an editor of the magazine. After Chambers died in 1961, Reagan said Chambers had taught him the "bitter truths" about communism.

The award, which Reagan presented to the recipient's son, John Chambers, cited him as "the focus of a momentous controversy in American history that symbolized our century's epic struggle between freedom and totalitarianism . . . in which the solitary figure of Whittaker Chambers personified the mystery of human redemption in the face of evil and suffering."

Chambers' identification of Hiss before the House Un-American Activities Committee led to a slander suit by Hiss, who after two trials was convicted of perjury for lying about his espionage activities and was sentenced to prison.

Chambers was one of three persons to receive the award posthumously. The others were assassinated Egyptian president Anwar Sadat, lauded as "a soldier whose greatest acts of courage came in pursuit of peace," and black baseball star Jackie Robinson, whose "courage opened the door of professional sports to all Americans."

Reagan departed from his prepared text once, when he presented a medal to his longtime Hollywood friend, actor James Cagney, saying, "As a great star at the same studio where I started, he was never too busy to hold out a hand to a young fellow just trying to get under way."

Cagney, 84, had tear-filled eyes as he accepted the award and a kiss from Nancy Reagan.

The other recipients were Sen. Howard H. Baker Jr. (R-Tenn.), economist Leo Cherne, heart surgeon Denton Cooley, singer Tennessee Ernie Ford, NATO commander Gen. Andrew Goodpaster, ballet promoter Lincoln Kirstein, western novelist Louis L'Amour, writer-theologian Norman Vincent Peale, Eunice Kennedy Shriver and Hector P. Garcia, a physician and first national chairman of the American G.I. Forum, a veterans' organization of Mexican-Americans.