Gen. Paul Xavier Kelley, who retires as commandant of the Marine Corps next week, said yesterday that American mothers who work and send their children to "faceless" centers rather than stay home to take care of them are weakening the moral fiber of the nation.

The four-star general also assailed the outlawing of school prayer and said it has weakened the moral fiber of American youth.

"I know I'm going to walk in a very, very tender area," Kelley said during a farewell breakfast with reporters, "but I'm going to walk there anyhow.

"Fifty percent of the mothers today work. And that means that a number of our children are not getting the kind of upbringing in their home that you and I had. Instead, their moral upbringing is being dictated by some nameless, faceless child-care center. A lot of people aren't going to like that remark, but I'm going to say it anyway.

"And then, you won't like this remark," Kelley continued: "We took God out of schools."

The commandant's statements were in response to questions about whether Marines in Moscow allegedly trading national secrets for sex and Marine Lt. Col. Oliver L. North's invoking the Fifth Amendment before Congress indicated a lack of moral fiber in the Corps he has commanded for the past four years. He replied that moral deterioration is nationwide.

"We are approaching in this country the fundamental problem with the moral fiber of the future youth of America," Kelley said. "I've made a recommendation that {since} we've got an institute for everything else -- we've got an institute for AIDS {acquired immune deficiency syndrome} and all these things -- why not have an institute for patriotic values."

He recommended that a group of American scholars be convened "to examine the {moral} problems of this country . . . " and develop solutions.

"It's there," he said of national moral deterioration. "It's latent. It's sort of waiting to happen. That's philosophical, for what it's worth. But I've thought about it for an awful long time."

Kelley, 59, grew up in Boston, graduated from Villanova University and in 1950 was commissioned a Marine second lieutenant. He was a decorated battalion commander in Vietnam and became the nation's top-ranking Marine on July 1, 1983. He will be succeeded next month by Lt. Gen. Alfred M. Gray Jr., 59, whom the Senate yesterday confirmed on a voice vote.

Kelley's friends have said he still grieves over the killing of 241 service personnel, mostly Marines, at Beirut airport in 1983 by a terrorist who detonated a truckload of explosives. Kelley has been especially wary since then, his associates said, of committing U.S. forces without a set of clear objectives. He said yesterday, however, that he supported President Reagan's decision to send U.S. warships to the Persian Gulf to escort 11 Kuwaiti tankers, calling the operation a "prudent risk."

Asked whether he regarded North "a hero or a bum," or if he objected to North wearing his Marine uniform while invoking the Fifth Amendment, Kelley replied that North left the Corps for White House duty in 1981 after compiling an "outstanding combat record." Since 1981, Kelley said, "he hasn't belonged to me." Therefore, Kelley said, he could not judge North's performance.

Kelley said it was North's "call" on whether to wear his uniform while testifying before Congress. "My problem," he said in regard to North invoking the Fifth Amendment, "is that 41 years ago I put up my hand and swore to defend the Constitution of the United States against all enemies, foreign and domestic. And I didn't say except when Ollie North takes the Fifth Amendment. He has a right as a serviceman to take the Fifth Amendment. I am not presuming from that that it is any indication of guilt or innocence."