A draft of the Reagan administration's first statement on national urban policy, which concludes that federal aid has led to the decay of American cities and should be curtailed, landed with the force of a bombshell at the U.S. Conference of Mayors' annual meeting today.

The White House, seeking to repair the damage wrought by published accounts of the draft policy, said that President Reagan had ordered it back to the drawing board "for more research."

But the nation's big-city mayors--two-thirds of whom are Democrats and nearly all of whom are strapped by domestic budget cuts and the worst economic conditions in 50 years--seized on the urban policy draft as another sign that the president has turned his back on the nation's poor.

The mayors quickly drew up, and passed by voice vote, a resolution urging Reagan "to reject the philosophy, approach and contents of this report." But in the face of criticism from Democrats, Republican mayors were muted, presumably because Reagan's assistant for intergovernmental affairs, Richard S. Williamson, disavowed the draft.

"This statement is a declaration of war," said Richmond, Va., Mayor Henry L. Marsh III. "I didn't believe the administration would even put out a trial balloon recommending elimination of virtually all the policy and programs for cities."

"Even as a draft statement it shows the kind of mentality we're dealing with," said San Francisco Mayor Dianne Feinstein. "I find it unbelievable." And Syracuse, N.Y., Mayor Lee Alexander said: "The timing almost seems to draw a battle line between mayors of the cities and the White House."

Williamson was engulfed by hostile questions about the draft policy when he appeared before the mayors' resolutions committee.

The document "has not been accepted as the position of the president nor has he embraced it," Williamson said. "It is not administration policy." The draft statement was leaked to The New York Times, which published accounts of it in Sunday's editions.

Williamson said the draft statement, described as a volume more than an inch thick, first came to Reagan's attention last Friday at a 75-minute session of the Cabinet Council on Human Resources. The statement was written by a "mid-level working group" that included E. S. Savas, assistant secretary of housing and urban development for policy development and research, and Robert B. Carleson, special assistant to the president for policy development, Williamson said.

During the session Friday, Reagan was given only a general view of the draft urban policy, Williamson said. "We didn't even get into the language that could be termed as inflammatory," he said, adding that he regarded some wording in the document as "unfortunate."

Williamson said the president and White House counselor Edwin Meese III then asked for "more research" into some subjects they thought were omitted from the report, including the problems of crime in the cities, education and training for unemployed teen-agers, and helping cities with their deteriorating "infrastructure," such as roads and bridges.

Williamson noted that Reagan has previously rejected some of the specific suggestions in the urban policy statement, such as elimination of general revenue sharing for cities. But in response to questions, Williamson said Reagan still believes that federal aid has not solved the problems of the nation's cities--which is the central message of the draft urban policy statement.

A portion of the draft, dated June 3 and made available here today, said: "The result of these prior efforts has been excessive dependence of city governments and city dwellers on the federal government with a . . . loss of local control and a loss of influence over their destinies."

The mayors have heard this from Reagan before, but they were particularly incensed by one section in the document which concludes that 20 years of federal aid transformed local officials "from bold leaders of self-reliant cities to wily stalkers of federal funds."

An incredulous Feinstein responded that, "I think Americans would much rather use their money for local services--things they see every day--than for massive programs in Washington."

By law the administration must file an urban policy statement with Congress every two years. This would be the first such statement drafted by the Reagan administration.

The draft urban policy statement takes an approach to city troubles markedly different from those of Democratic and Republican presidents of the last two decades. It calls for turning away from billions of dollars in federal aid and suggests that "greater self-reliance is essential to the long-run good health of our cities."

For example, the draft document concludes that "government intervention" has been "carried to an excess" in programs designed to help people. "In particular, many poor households have become virtual wards of government agencies.

"This is sometimes necessary to maintain the dignity and health of the truly helpless; however, too often the existing, well-meaning welfare system undermines personal ambitions for self-betterment," the policy declares. "The so-called urban underclass is a particular victim of such policies."

The document goes on to suggest that American cities should rely more heavily on families, neighborhoods, businesses and civic associations than Washington for help. ". . . One must not lose sight of the fact that the family is the original department of health, education, welfare, housing and human services," the draft statement says.

It suggests that "unless a clear and direct national purpose is served," the federal government should disengage itself from many of the traditional urban aid programs.

Talk of the draft statement took on a partisan tone and dominated the mayors' sessions today. Alexander, the Syracuse mayor and president of the National Conference of Democratic Mayors, distributed a report card giving Reagan a failing grade on most aspects of urban policy. "Yesterday, some of us thought this might be too severe," he said, "but now some of us think it's not severe enough. This administration has no urban policy."

"I am apprehensive about what the president can do to my city," he said. "But now we don't have to worry, he's doing it to all our cities."