In a solemn funeral service attended by some of the country's leading politicians and diplomats, statesman W. Averell Harriman was eulogized today as "one of the last great giants of our day."

Harriman was 94 when he died Saturday at his Westchester County estate of kidney failure and pneumonia after serving his government for more than 50 years.

"A great and extraordinary person has departed this life, one of the last great giants of our day . . . a man in love with history and the making of it," Episcopal Bishop Paul Moore Jr. told about 750 mourners at St. Thomas Episcopal Church on Fifth Avenue. "To recall his life story, even in summary, is a staggering experience."

Heir to a railroad fortune, Harriman advised five Democratic presidents, from Franklin D. Roosevelt to Jimmy Carter, and was Roosevelt's ambassador to Moscow during World War II and to the Court of St. James's under President Harry S Truman. Many of those attending the service had served in the same administrations.

"He spread himself across the whole Earth -- Asia, Africa, Russia, again and again. He lifted up a broken Europe to her feet. He was tough yet balanced with the most abstruse problem, the most wily adversary," said Moore, describing Harriman as a man "who was there on every dangerous lurch of events in this tumultuous century of ours."

He negotiated with Leon Trotsky in 1926 and had met with every subsequent Soviet leader through Yuri Andropov; he cemented a wartime alliance with Winston Churchill and Josef Stalin; he laid the groundwork for the Marshall Plan that helped restore postwar Europe; he helped formulate the nuclear test ban treaty in 1963; and he advised Carter on the strategic arms limitation talks, SALT II.

"He always knew that to negotiate successfully, both sides had to be willing to give," former senator and secretary of state Edmund S. Muskie (D-Maine) said outside the church, recalling a visit he made to Moscow with Harriman in 1971. "He was a marvelous man."

Ushers at the hour-long ceremony included five of Harriman's fellow advisers to President John F. Kennedy -- former national security adviser McGeorge Bundy, Washington lawyer and former defense secretary Clark Clifford, Harvard economist and former ambassador to India John Kenneth Galbraith, presidential adviser and historian Arthur Schlesinger Jr. and speech writer Theodore Sorensen -- and television broadcaster John Chancellor, academician Marshall Shulman and former secretary of state Cyrus R. Vance.

"What I most admired about him was that he had all this wealth and yet he served his country for 50 years. He didn't have to do all this," said former vice president Walter F. Mondale after the service. "He was as tough as nails and kept up all that energy until his 80s, setting a good example for all of us."

U.N. Ambassador Vernon A. Walters represented President Reagan at the service, which also was attended by New York Gov. Mario M. Cuomo, Mayor Edward I. Koch, former secretary of state Henry Kissinger and former mayors Robert Wagner and John Lindsay Jr.

Among guests from the Senate were minority leader Robert C. Byrd (D-W.Va.), Joseph R. Biden (D-Del.), Dale Bumpers (D-Ariz.), Christopher J. Dodd (D-Conn.), Edward M. Kennedy (D-Mass.), Gary Hart (D-Colo.), Howard M. Metzenbaum (D-Ohio), Daniel Patrick Moynihan (D-N.Y.), Claiborne Pell (D-R.I.) and Paul Sarbanes (D-Md.).

Despite swelteringly humid weather, crowds lined Fifth Avenue six deep to pay their respects as the former New York governor's seven grandchildren and his godson, band leader Peter Duchin, carried the flag-draped coffin down the steps of the church while the choir sang the "Battle Hymn of the Republic."