Federal Reserve Board Chairman Alan Greenspan yesterday issued an unusual public warning to the Reagan administration to stop pressing the Fed to lower interest rates and boost the nation's money supply. Otherwise, he said, Fed policymakers might have to demonstrate their independence by tightening credit and raising rates even if economic conditions did not call for it.

Greenspan's statement, made at a Senate Banking Committee hearing, followed recent criticism of Fed policy by Beryl Sprinkel, chairman of the Council of Economic Advisers, and other administration officials suggesting that the Fed increased the risk of an economic slump this year by keeping too tight a rein on credit during 1987.

Senate Banking Committee Chairman William Proxmire (D-Wis.) said at the hearing that he was "troubled by the extent of the political pressure being put on the Fed by the Reagan administration in this presidential election year. I think it is both deplorable and counterproductive for the Reagan administration to pressure you as much as they have in recent months."

Proxmire added, "Ironically, when the administration badgers the Federal Reserve they may make it harder for you to ease monetary policy than would be the case if they were silent. Even if you have eased money for sound technical reasons, you will certainly be accused of bending to political pressure. Insinuations of political manipulation harm both your reputation as chairman and the reputation of the Federal Reserve."

Greenspan appeared to be particularly angered by a letter sent last month to Fed policymakers by Michael R. Darby, assistant Treasury secretary for economic policy. The letter, suggesting that a decline over the past year in one measure of the nation's money supply was signaling either a recession or a substantial slowing of economic growth, was received by most Fed governors and Federal Reserve bank presidents not long before they met this month to set monetary policies for 1988.

"After Michael Darby's letter, I objected quite strongly," Greenspan told the committee. Darby apparently was not aware of the implications of sending such a letter prior to a meeting of the policymaking group, the Federal Open Market Committee, he said.

"I am reasonably sure such actions will not reoccur in the future," Greenspan said.

A Fed official confirmed that Greenspan had raised his objections with Treasury Secretary James A. Baker III and received assurances from him that Darby would not send such a letter again.

Neither Baker, who was in South Korea, nor Darby was available for comment. A spokeswoman for Sprinkel said, "The chairman has testified that he is very pleased with the Fed's targets {for money supply growth} for 1988, and there is absolutely no disagreement between them as to what the appropriate rate of money growth should be."

Greenspan said Fed policy has "not been unduly influenced by the administration," and he is not sure how intent administration officials are about putting pressure on Fed. However, he said, "They have."

Greenspan then said he hopes central bank officials do not become so concerned "about our responding to political pressure that we would feel forced to do the opposite and take steps" that would run counter to the best judgment of Fed policymakers in order to show the Fed was still able to act independently.

Maintaining an appearance of independence from political pressure is a longstanding concern at the Federal Reserve. In the present context, Fed officials fear that inflationary pressures could grow if financial markets decided the central bank was pursuing an easier monetary policy to make sure the economy keeps growing between now and the election. Such a change in expectations about inflation likely would lead to higher long-term interest rates regardless of what the Fed was doing to influence the short-term interest rates over which it has more control, according to Fed officials.

Darby's letter was accompanied by figures and several charts which, he said, demonstrated that a downturn in the money measure known as M2, adjusted for inflation, was followed by a recession or a substantial slowdown about a year later. The letter made no specific policy recommendation.

One recipient of the letter, Gary H. Stern, president of the Minneapolis Federal Reserve Bank, said, "I had several reactions. I was somewhat surprised. I did not recall receiving something like that in the past. On the other hand, I was not shocked because Mike Darby is something of an academic. This was something he was following, something he wanted to call attention to."

Stern said he gave the letter and data to members of his research staff, who were initially impressed by the apparent predictive power of changes in real M2. However, he said, as of yesterday they had not been able to reproduce the charts starting from raw data, possibly because of a lack of information about exactly how Darby divided the money numbers by the consumer price index to determine a "real" value for M2.

Another Fed official suggested that part of Greenspan's strong reaction to the letter was the context in which it was sent. The letter, the official said, came on the eve of one of the Federal Open Market Committee's most important meetings of the year; it virtually coincided with a central bank decision, confirmed Tuesday by the chairman, to slightly reduce short-term interest rates to counter a slowing in economic activity; and it followed sharp public and private criticism of Fed policy by Sprinkel.

At yesterday's hearing, Greenspan again rejected in strong terms complaints from Sprinkel that money growth was too slow last year, even though it fell below the Fed's own target ranges. "If we had tried to stabilize money, we would have destabilized the economy," he said.

Greenspan reiterated the central bank's position that the relationship between various measures of the money supply and the economy and inflation have become so tenuous, after an earlier period in which they appeared much tighter and more stable, that they can for now not be used as a guide for setting policy.