A week ago, on his way to a candidate interview with the American Jewish Committee, Mayor Marion Barry heard that many people who were to attend the meeting would be late. They were at a rally in Lafayette Park supporting Israel's invasion of Lebanon, called to coincide with Israeli Prime Minister Menachem Begin's visit with President Reagan.

Barry ordered his driver to go to the park and, in what is being described as a major gaffe by people in his own camp, Barry gave an impromptu speech in support of Israel.

The speech--and his appearance at the rally--did not go over well in many parts of Washington.

What did Barry say?

"There is no way anyone could ever leave Israel after seeing the atrocities of 6 million people being murdered and assassinated and killed, that you could come back without a strong sense that you had to do all you could to make sure that the Israelis and the people of Israel lived in peace and were secure at their borders," he said.

Barry, who visited Israel in 1980, added, "I'm here to offer my own strong support for making sure that that happens. That in Lebanon you get a democratically elected and a free government. A government that believes in peace for itself and not war against Israel . . . and so I offer my support and the support of all the citizens of the District of Columbia, because myself, being a minority, I understand very clearly that if one of us is not free, no one is free."

The phones rang off the hook with angry calls at Barry's campaign headquarters and at the District Building. WJLA-TV (Channel 7), which broadcast part of the mayor's speech, also was deluged with calls protesting the mayor's pledge of support for Israel from the people of Washington.

The speech apparently was very unsettling to younger blacks who identify with Barry for his civil rights activities and are now active in national and international politics as well as the local political arena.

"I don't understand either the statement, the logic or even the political reason for making such a statement," said Randall Robinson, executive director of TransAfrica, a predominantly black lobbying group interested in U.S. policy in Africa. "On this question of Israel's invasion, we find support for Israel confined to the Reagan administration and Marion Barry. That is truly extraordinary."

"To have Marion Barry support this outrageous tactic on Israel's part is lamentable to say the least," he said. "To suggest that he is representing the people of D.C. is even further misleading."

There were other dissenters to the mayor's statement. Attorney James G. Abourezk, a former Democratic senator from South Dakota and a leading spokesman in Washington for Arab causes, said that even if Barry intended his remarks to compensate for a statement made during the Atlanta child killings (Barry said at one point that if the children had been Jewish instead of black, the federal government would have reacted quicker) it still was improper to involve the people of the city in his political scheme.

"It's very poor political judgment on his part and, more importantly, to voters it's evidence of poor moral judgment," said Abourezk, son of an immigrant from Lebanon.

"The majority of the people in D.C. don't support one country invading another, not white people or black people. I don't think the majority of Jewish people support that. Why would he commit the whole District to something like that? It was in very bad taste."

Even angrier was Bob Joseph, president of the National Association of Arab Americans. He said Barry was being opportunistic in using the war to benefit him in his reelection race.

"I challenge the mayor to get on an airplane and fly to Lebanon," Joseph said. "I want him to see the sight of destruction, to smell the smell of death, to see the bodies, the children killed or walking without arms or legs and come back and tell us that whatever the reason, the people of D.C. support what is happening."

Not all of the reaction to Barry's speech was anger. Hyman Bookbinder, head of the Washington Chapter of the American Jewish Council, said the spectators in the pro-Israeli crowd were pleasantly surprised the mayor committed himself to supporting Israel.

"Among some friends of Israel," Bookbinder said, "they are displeased that he brought it into local politics. But Israel is the number one issue for Jewish people. There are other issues, but Israel is very important."

The other major candidates for mayor--Patricia Roberts Harris, Charlene Drew Jarvis, and John Ray--said Monday night at a forum at the Washington Hebrew Congregation that they believed the crisis in Lebanon should not be made an issue in a local political race, although all three said they were supporters of Israel.

Barry explained his remarks this way: "Just like a lawyer going to court where murder is being talked about, that doesn't mean you support murder. I support the right of all people to live in safety and without fear."

He said he always has opposed war, including the Vietnam War. When asked why he made his statement in the name of all citizens of Washington, he said: "I'm the mayor of all the people, and I believe the people of the District of Columbia oppose any effort against Israel."

The Jewish Community Council of Greater Washington and the Zionist Organization of America said they organized the rally to express solidarity with Israel, to press for withdrawal of all military forces from Lebanon and to work for a free, democratic Lebanon.

Park police estimated about 300 people attended the rally, but organizers said several times that number took part.