Nearly 300 friends and relatives of Alex Singer filed into the Israeli Embassy yesterday and sat in quiet grief through a memorial service that rang with praise for the former Chevy Chase schoolboy who followed his conscience to Israel two years ago and died last month fighting in Lebanon.

"Once you knew what the moment demanded of you, you did it," one friend told the gathering, recalling Singer as an introspective young man and a passionate Zionist who did what his heart demanded. "You always did, despite the cost -- whether it meant moving to the Israeli state with the Jewish people, or running into terrorist gunfire."

Singer, a 1980 graduate of Bethesda-Chevy Chase High School and a lieutenant in the Israeli army, was one of three soldiers killed in an ambush in southern Lebanon Sept. 15, his 25th birthday. The toll was Israel's largest in a single incident since most of its troops were withdrawn from Lebanon in 1985.

An Israeli army spokesman charged that the guerrillas were intent on infiltrating Israeli territory.

One by one yesterday, friends and relatives faced the gathering amid the soft light of an embassy reception room and recalled the intellectual journey that preceded Singer's decision to emigrate in 1985.

"My father taught me that a hero is not someone who thinks moral thoughts," said Larry Kelemen, a friend from California. "Many people think moral thoughts. A hero is one who makes his moral thoughts into actions. Dammit, Alex, you were a hero."

Alex Singer was 11 years old when, in 1973, he first saw Israel. Max Singer, then an analyst for a research group in New York, took a sabbatical in Jerusalem with his wife and four sons. One of them, Daniel, 22, is now an Israeli paratrooper. They stayed nearly four years before returning to settle in Chevy Chase.

After finishing his sophomore year at Cornell, Alex Singer entered a summer program of Judaic study at the Brandeis-Bardin Institute in Southern California. And the next academic year, after enrolling in the London School of Economics, he used much of his free time to explore Jewish communities in England, Spain, Italy and the Soviet Union.

"He was a traveler in the classic sense," his mother Suzanne said in an interview. "He wanted to learn and experience the world. He was by himself most of the time. He would go to a pub or a train station -- wherever he went, he would meet people and learn. He had a talent for that."

The long, thoughtful letters he mailed home, friends said, reflected a young man troubled by a deep-rooted insecurity he sensed among the Jews whom he visited during his travels -- Jews assimilated in other cultures.

"Zionism is still valid, because Judaism is still threatened," he wrote to Kelemen, who read the letter to the gathering. "But assimilation has replaced anti-Semitism as the true danger. And the Jewish state, which was once an answer to dangerous anti-Semitism, is now an answer to assimilation."

Back at Cornell, he compiled more than 200 pages of his correspondence for his senior year

thesis, "Letters from the Diaspora." Convinced of the need for a secure Jewish homeland, he took Israeli citizenship in February 1985, eight months after graduating.

"The whole world looks at us now just as they did in antiquity," said Benjamin Netanyahu, Israel's ambassador to the United Nations and a friend of the Singer family. "They look at us and say, 'Where do these people come from? Where do they find the power to defeat their enemies, so more numerous than them?' I'll tell you where we find the power. In antiquity, as in modernity, we find the power in Alex Singer."

In the military, as through much of his life, Singer kept a diary. And after a 50-mile endurance march that ended at Jerusalem's Western Wall and concluded his basic training in July 1985, he recalled the final miles.

A friend read the passage yesterday:

"The views, of course, were another joy, as they always are when one enters Jerusalem. But on foot, to moonlight, through valleys of pine, the entrance is especially special and moving . . . . During the harder moments, I would think of the end ahead of me, of running the last steps to the Wall, and tears would come to my eyes and I would have strength. And in the end, when I was about to take those final running steps to the end of basic training, the tears came to my eyes. Tears of joy and pain, but mostly of joy."