U.S. Circuit Judge Harold Leventhal, 64, known for his well-crafted opinions resolving complex appellate litigation, died at George Washington University Hospital yesterday following a heart attack.

Judge Leventhal, who had been a member of the highly visible and often controversial appellate bench here since being appointed by President Lyndon B. Johnson in 1965, was playing tennis around noon with one of his law clerks when he was stricken.

U.S. Circuit Chief Judge J. Skelly Wright, upon being informed of Judge Leventhal's death, spoke of "the brilliance of his legal mind. No one -- either friend or foe -- would challenge that. He also had the industry to do something about it."

Judge Leventhal had a reputation as a dry, keen wit, as well as one for indefatigable work. Wright described his colleague's opinions as "path-breaking in the law," and noted that he was one of the court's most prolific legal writers.

He had written widely both on and off the court in numerous areas, especially in the field of administrative law. But reports of his opinions in his 14 years on the bench show him also as the author of far-reaching rulings in immigration law, libel law, criminal law, civil liberties and other fields.

As recently as Monday, he had sat on a three-judge appellate panel that approved a Carter administration ban on demonstrations across from the White House concerning the Iranian hostage situation.

U.S. Circuit Judge Carl McGowan, a classmate of Judge Leventhal at Columbia University Law School in the mid-1930s, said Judge Leventhal had a "very lively legal mind" that saw "all kinds of angles.

"He was a very effective, very productive member of this court," McGowan said. "His death is a great blow to the court as an institution."

Judge Leventhal was a native of New York. He graduated from Columbia College in 1934, and received his law degree from Columbia in 1936. He was first in his class and editor-in-chief of the Columbia Law Review.

He then became a law clerk to Supreme Court Justice Harlan Fiske Stone, who traditionally hired as his clerk the top Columbia student. Judge Leventhal worked briefly at the Justice Department after that, McGowan said, but then again became a law clerk for newly appointed Supreme Court Justice Stanley F. Reed upon Stone's recommendation.

Judge Leventhal later returned to the Justice Department, and was assistant general counsel for the Office of Price Administration during the early years of World War II. He then became a lieutenant commander in the U.S. Coast Guard, and was assigned to the legal staff at the Nuremburg war crimes trials.

After returning to Washington, he began practicing law with the firm of Ginsburg and Leventhal and its successor firms. At various times he also was chief counsel of the Office of Price Stabilization and executive officer of the Hoover Commission task force on independent regulatory commissions.

Judge Leventhal was a visiting lecturer at Yale University Law School on the topic of "Regulated Industries" from 1957 to 1962, and served as an attorney for, among others, the C&P Telephone Co. here. He was also instrumental in winning a major consumer victory in the early 1960s when he and two other lawyers had a D.C. Transit bus fare increase ruled illegal. h

He became general counsel to the Democratic National Committee in 1952, and in 1964 played a major role in the seating of the Mississippi Freedom Party delegation. In an editorial upon his nomination to the bench, The Washington Post said Judge Leventhal's work for the Democratic committe "was statesmanly as well as politic and served his country as well as his party. He will bring learning, sensibility and a richly reflective mind to the bench."

As a judge, he usually was aligned with the liberal wing of what became known as one of the most liberal courts in the country. However, he often wrote his own separate opinions explaining why he sided with the majority.

He served on the court here with Warren E. Burger before the latter's elevation to chief justice of the United States. Burger said he viewed Judge Leventhal as "my highly respected colleague. He was a lawyer of extraordinary talent and his work as a judge was of the same high caliber. He will be sorely missed."

Judge Leventhal wrote about topics as varied as ordering free transcripts of their trials for indigent defendants and ruled that the urban renewal agency that rebuilt South west Washington must help uprooted businesses find new locations.

He warned Congress in one opinion that the court would begin reversing criminal convictions unless speedy trial laws were passed. He said in another ruling that Congress never gave the State Department the power to deny passports to travelers because they would not promise to stay out of restricted countries.

He ruled in 1971 that president Richard M. Nixon acted within the law in setting up his wage-price freeze program. But in the same period of the early 1970s, he also wrote opinions critical of the Nixon administration's handling of massive antiwar protests.

Despite his high volume of work on the bench, Judge Leventhal was known as a private defender of the English language, and once wrote a letter to the editor of The Post discussing the increasing use of the word "parameter" and its interchangeability with the world "perimeter."

Attorney General Benjamin Civiletti said yesterday that Judge Leventhal's death "deprives me, the legal community and the community at large of a humane friend and a distinguished scholar. Judge Leventhal combined a keen legal intellect with a deep sensitivity to human rights"

Judge Leventhal is survived by his wife, Kathryn, of Washington, and two children, Philip H. J., a student at Northwestern University Law School, and Anne K., a student at Susquehanna University in Selinsgrove, Pa.