Attorney General William French Smith has accused the Federal Emergency Management Agency of trying to hustle a proposal through the White House that would greatly expand FEMA's power, making it a virtual "emergency czar" with authority over other federal agencies.

Smith lodged his criticism in a letter Aug. 2 to Robert C. McFarlane, assistant to the president for national security affairs. The Washington Post obtained a copy of the letter.

FEMA sent the White House a revised version of Executive Order 11490 this summer for President Reagan to sign, Smith wrote. The order spells out federal agencies' roles in a national emergency.

Smith said FEMA had proposed revisions that would expand its authority over other federal agencies. The agency, he said, also has moved recently to broaden the definition of "domestic emergency" so it could become involved in routine domestic law enforcement. Smith didn't cite examples of FEMA police work. A spokesman said Smith would not comment on his letter.

Smith urged McFarlane to delay issuing the revised order until FEMA's role could be clarified and limited.

FEMA was created in 1979 when President Carter consolidated five small, separate disaster programs. It currently develops civil defense plans, administers federal flood insurance, trains state and local officials in emergency planning, trains firefighters and advises the White House on distributing disaster aid.

In his letter, Smith said that he became aware of FEMA's proposed revisions when the Office of Management and Budget sent the Justice Department a copy of the changes for routine legal review. That review isn't finished, Smith said, but he decided to object to the changes on policy grounds.

Smith said it frequently had been difficult to "separate the coordinating responsibilities of FEMA from the operational responsibilities of other departments and agencies" in case of a national disaster.

"I believe that the role assigned to FEMA in the revised executive order exceeds its proper function as a coordinating agency for emergency preparedness," Smith said.

"Recent FEMA continuity of government plans feature layers of FEMA operational personnel inserted between the president and all other federal civil agencies," he said. "Its mobilization exercise scenarios continue to assign FEMA the responsibility of representing the Department of Justice and other Cabinet agencies at meetings with the president and the National Security Council during national security emergencies."

A number of Cabinet members had "repeatedly raised serious policy and legal objections to the creation of an 'emergency czar' role for FEMA," Smith wrote. "In spite of these objections, the draft executive order seems to legitimize FEMA's authority beyond that of a coordinating agency."

FEMA General Counsel George Jett said the letter surprised FEMA officials. He said the revisions were intended to clarify and update FEMA's role, not to expand its authority.

Other FEMA officials played down the incident, describing it as a misunderstanding between two agencies negotiating an executive order. But a copy of an internal memorandum written Aug. 27 by Jett and obtained by The Post shows that he, at least, considered Smith's letter a major threat to the agency.

Jett told Bernard A. Maguire, associate director for national preparedness programs, that FEMA "should not circulate at this time any proposed documents which related to FEMA's legal authority," including the proposed revisions.

"Until the present policy impasse with the Justice Department can be resolved, FEMA runs the grave risk of seeing its prerogatives diminished rather than enhanced . . . ," Jett warned.

"The potential for mischief created by the present atmosphere is especially serious for FEMA because so much of its legal authority in the national security preparedness area consists of inherent presidential authority that . . . can be changed with relative ease and can be influenced by temporary forces at work in the system."

This is the second time in the last few weeks that FEMA has come under fire.

Last month, FEMA's No. 3 official, Fred J. Villella, resigned amid charges of mismanagement levied by a House subcommittee. Among other things, subcommittee investigators alleged that the official had spent $170,000 of agency funds for improvements to a FEMA building so he could use it as his private residence.