In a highly unusual, if not unprecedented, step, Gen. George S. Brown, chairman of the Joint Chiefs of Staff, called a meeting of retired generals and admirals to explain his reasons for supporting the proposed Panama Canal treaties.

Pentagon spokeman Thomas B. Ross confirmed yesterday that Brown held the meeting at the National War College at Ft. McNair Monday afternoon, but said neither President Carter nor Defense Secretary Harold Brown ordered the general to do so.

Carter is mobilizing his administration to fend off attacks on the proposed treaties, which would turn the canal over to Panama by the year 2000.

While leading the political offensive, Carter has enlisted a wide range of present and former government officials to help him win what the White House concedes is an uphill fight.

Mail coming into the White House on the treaties "is overwhelmingly opposed." presidential spokeman Jody Powell said yesterday.

Dean Rusk, former President Kennedy's Secretary of State, was among the influential citizens and governors invited to the White House yesterday to hear Gen. Brown, Carter, Deputy Defense Secretary Charles W. Duncan Jr. and Panama treaty negotiators Ellsworth Bunker and Sol Linowitz.

Afterward, Rusk endorsed the treaties and warned that if the Senate rejected them it would be "foolhardy" to reject the possibility that guerrilla war are might break out in Panama.

The Carter administration's political offensive has been waged through frequent appearances on television talk shows as well as formal briefings.

Pentagon officials said yesterday that they had not been able to find a precedent for the chairman of the Join Chiefs - the nation's highest ranking military offier - to call in retired officers to brief them on a controversial presidential proposal.

Ross said it would be fair to characterize the briefing of 75 retired three and four-star generals and admirals in the Washington area as unusual Gen. Brown and Secretary Brown had discussed the importance of getting the proposed treaties approved, Ross said.

The Senate must approve both proposed treaties. The Carter administration has not decided whether not to push for a vote this year or next.

Retired military officers, if they speak out in opposition to the treaty, would strengthen the assaults conservative politicians already are making against it.

One retired admiral who attended the Friday session at McNair said his reading of fellow officers: "Given the situation we were in, what was the alternative" to turning control of the canal over to Panama?

The admiral said he interpreted that remark to mean that the joint chiefs had no choice but to go along with the firm position taken by their commander-in-chief, President Carter.

Pentagon spokesmen said that what the general has been trying to convey in such comments is that, given the rising expectations of the Panamanians, there is no better alternative than give them control of the canal with the proviso that the United States can use it.

The formal position paper of the Joint Chiefs of Staff endorsing the proposed treaties includes these statements: "United States military interests in the Panama Canal are its use, not its ownership . . . In a hostile environment, even under the current treaty, continued operation of the canal cannot be guaranteed." The Panamanians would have a "vested interest in the continued operation of the canal" once they control it. "The preferred protection and defense of the canal would be in conjunction with a friendly Panama."

In his appearance at the White House briefing yesterday, Gen. Brown rejected as "vicious accusations" that the chiefs were supporting the canal treaties as an "act of loyalty to the President."