The Reagan administration sought to wash its hands today of a draft statement on national urban policy that concludes that federal aid has weakened American cities, but doubts and criticism lingered among the nation's big-city mayors.

Secretary of Housing and Urban Development Samuel R. Pierce Jr. told the opening session of the U.S. Conference of Mayors that alarm over the policy statement "is all a little premature." Pierce described the document as only "a draft done by the staff, and as far as I and the administration are concerned, it will remain a draft."

Talking with reporters later, Pierce said he had not approved the urban policy statement even though it was largely written by one of his chief deputies and a summary of it was discussed with President Reagan at a White House meeting last week. Pierce, trying to calm the uproar among the mayors, said the policy statement would be revised before it is sent to Congress.

The aide who wrote the report was E. S. Savas, assistant secretary for policy development. Today, Savas' deputy, Roger Ahlbrandt, said he is resigning. Sources said Ahlbrandt strongly disagreed with the thrust of the urban policy draft and is returning to the University of Pittsburgh.

Reagan also tried to reassure the mayors that the statement does not reflect his thinking.

"The president's reaction is that this is a draft of an urban policy statement and it is only that: a draft," said deputy White House press secretary Larry Speakes. "It was prepared at staff levels. It has not been approved nor has it been adopted as administration policy."

Speakes said Reagan pledged "that the federal government will not abandon its partnership with the nation's cities."

Speakes was even blunter in responding to reporters' questions about the report, referring to it at one point as "something that floated out of HUD" and saying at another that it would not be approved "in its present form."

Even as the administration sought to defuse the issue and build support among the mayors for Reagan's proposed enterprise zone legislation, the urban policy statement continued to provide a catalyst for criticism of the administration's approach to the poor and the cities.

Felix G. Rohatyn, the New York investment banker and chairman of the Municipal Assistance Corp., received a standing ovation from the mayors for a speech in which he asserted: "Drop dead in draft form is not much better than drop dead in final form."

The draft urban policy statement that the administration is required by law to submit to Congress suggests curtailing many federal aid programs of the past on grounds that they had led to the decline of the nation's cities.

Richard S. Williamson, special assistant to the president for intergovernmental affairs, said today that Reagan, at a White House meeting Friday, was briefed from a 16-page summary of the 40,000-word report. At that time, Reagan asked that the urban policy be sent back to a staff working group for more research on several points he thought were omitted, Williamson said.

Pierce, however, told reporters that what was discussed with Reagan was "different" from the draft policy statement that angered the mayors.