''He was really an outstanding man in every way in the congregation,'' said Rabbi Leonard Goldstein, who retired from Temple Beth El several months ago. ''If the charges against him are true, it will be a great shock to me.''

''What they said he did I can't believe,'' said Isaac Lebowitz, who has prayed daily with Mr. Tannenbaum for more than a decade. ''He is straight and charitable and kind.''

Mr. Tannenbaum's charities included the Wiesenthal Center, to which Mr. Tannenbaum donated $169 from 1979 through 1986, the center's records show. ''My father was in favor of going after Nazis,'' said Sonny Tannenbaum, one of Mr. Tannenbaum's three children. ''He never thought the Nazi-hunters would come after him.''

Living on Social Security after retiring from a dairy, Mr. Tannenbaum donated money in a drive to retire Temple Beth El's mortgage. He gave the synagogue a parochet - the curtain that covers the ark holding the Torah scrolls - that was embroidered with the names of his slain family members. On the door to his beige bungalow on Lawn Court, he hung a mezuzah - the small Biblical scrolls that indicate a Jew lives within.

While Mr. Tannenbaum told his immediate family and some of his closest friends that he had been a kapo in the forced-labor camp at Gorlitz in what is now East Germany, Federal authorities maintain that he kept this secret when he entered the United States in 1949 and again when he gained citizenship in 1955. And no one except his victims, the authorities allege, knew just how violent a kapo Mr. Tannenbaum had been - beating, whipping and kicking other inmates, with and without Nazi officers present. Maintains Innocence

''This is not a case brought simply because Mr. Tannenbaum served as a kapo,'' said Neal M. Scher, the director of the Office of Special Investigations. ''This was brought because he engaged in physical violence against inmates. It is based on a brutality that was unwarranted by any standard.''

There are indications, said Rabbi Hier of the Wiesenthal Center, that prisoners liberated from Gorlitz began hunting for Mr. Tannenbaum in Europe and overseas shortly after the war. At least one newspaper reported that the Federal case against Mr. Tannenbaum had been first prepared in 1979, although Mr. Scher declined to address the issue. Mr. Scher did say, however, that the current charges derive from eyewitness testimony.