Interior Secretary James G. Watt's new wilderness proposal was denounced by a key congressman yesterday as an attempt to "gut the nation's wilderness system."

Rep. John F. Seiberling (D-Ohio), chairman of a House Interior subcommittee, charged that a draft of Watt's proposed legislation "would be the most sweeping and devastating anti-wilderness bill I have ever seen."

Seiberling's opposition assures serious trouble and probable defeat for the legislation, which Watt previewed Sunday in a television interview as a bold administration plan to prevent mining and drilling in the nation's vast wilderness tracts now closed to industrial uses.

A draft of the legislation was also denounced by the Wilderness Society as a "wilderness destruction bill," and the organization's conservation director, Charles Clusen, accused Watt of engaging in a "massive public relations deception."

It could not be learned whether the draft of the legislation now circulating is the final version, which Watt has promised to send to Congress this week. Congressional sources reported yesterday that there are indications that it may be revised by Interior before being submitted formally.

Watt announced Sunday that he would propose to bar mineral drilling and mining in all wilderness areas until the end of this century, a goal embraced by conservationist organizations and many members of Congress. On Monday, several House members gave cautious approval to that concept, unaware of what the draft contained.

Under current law, there can be no more leasing of wilderness lands after Dec. 31, 1983. Watt's proposal would move that deadline up to the present, but it also would leave open the question of mineral development after the year 2000, a proposition that conservation organizations deplore.

The draft bill also would have the potential effect of opening to private mineral development many millions of acres now being studied for preservation as wilderness areas. Those areas are now shut off to developers, pending studies by Interior and action by Congress.

The so-called "release" of these study lands has been bitterly contested in Congress. Legislation introduced by Sen. S.I. Hayakawa (R-Calif.) would release to development millions of those acres unless Congress consigned them to a wilderness category. The administration endorsed that bill, but the measure has been stymied by strong opposition from conservationists.

Some of those wilderness study areas are on lands controlled by Interior's Bureau of Land Management. Current law leaves their future up to Congress, but the draft of Watt's new proposal would permit the administration to assign them for private development such as mining, drilling or timber-cutting.

Seiberling criticized each section of the draft, and called the legislation "deficient and unacceptable in every major respect."

"I have learned by now to look for 'fish hooks' in Mr. Watt's glittering proposals," he said. "This particular one appears to be an attempt by the secretary to portray himself as pro-wilderness when actually developing legislation to gut the nation's wilderness system."

Seiberling chairs the subcommittee on public lands and national parks, which has primary jurisdiction over all legislation affecting wilderness areas. It is to hold hearings on the new plan in mid-March.