Interior Secretary James G. Watt misused almost $9,000 in government and private funds to pay for two controversial private Christmas parties held at Arlington House, the General Accounting Office said yesterday.

As a result, Watt is under pressure to reimburse about $4,500 of the expenses personally.

The Interior Department "was not authorized to use either its operating appropriations or donated funds to pay for either" of the social events, the GAO report said. Although expenses for one of the parties may be paid from the remaining money in Watt's 1982 official entertainment budget, that money will be insufficient and about $4,500 should be paid "by the Interior officials who authorized the expenditures," in the GAO's opinion.

Interior spokesman Douglas Baldwin said the GAO report is "in error. We carefully researched the propriety and procedures of the Lee Mansion events before they occurred. We are confident everything was done correctly."

But Rep. Edward J. Markey (D-Mass.), chairman of the House Interior Committee's subcommittee on oversight and investigations, said that "since GAO indicated Secretary Watt broke the law, I expect him to come to a hearing scheduled for Friday with checkbook in hand, ready to reimburse the federal treasury." Both Interior officials and Arlington House volunteers are expected to appear at the hearing called by Markey.

Although invited, Watt has declined to attend the hearing, saying he does not want to be involved "in a media sideshow."

The parties, a cocktail party on Dec. 17 and a small breakfast hosted by Watt's wife Leilani three days earlier, were first criticized by volunteer guides at the Park Service-owned mansion as improper use of a national monument.

Arlington House, sometimes called the Custis-Lee Mansion, the home of Robert E. Lee overlooking Arlington National Cemetery, was closed to the public during regular visiting hours during the breakfast, and historic furniture was moved in and out of storage for the events. A large, heated tent was pitched on the lawn for the cocktail party, and the circular driveway, off limits to tourist traffic, was used by official limousines and other guests' automobiles.

At the time, Interior spokesmen described the parties as unofficial and denied that any of the agency's employes present were paid for their help. The GAO report says 28 Park Service employes worked a total of 166 hours, most of it overtime, to run the two parties, at a cost of $2,200.

According to the GAO report, the Dec. 17 cocktail party cost $6,921 and was attended by more than 200 high-ranking government officials, including presidential adviser Edwin Meese and members of the Republican National Committee. Watt initially billed $1,800 in Park Service labor costs to the Park Service, intending later to reimburse this from his official entertainment fund or from a fund composed of voluntary donations from nonprofit historical groups that sell literature in national parks.

Watt's stated intention to pay for the parties out of that voluntary fund drew fire from members of some associations that contribute to the fund, as well as from Markey, who asked the GAO to investigate.

Watt announced his intention this month to refund $2,000 in party expenses to the voluntary donations fund from his official entertainment fund.

The GAO said yesterday neither the Park Service funds nor the voluntary fund could be used to pay for the cocktail party as "neither the breakfast or the party were associated with any . . . goverment conference or meeting." The GAO found Watt's attempt to link the two events to Park Service purposes by arguing that party guests were free to tour the house and thus could become aquainted with its historic significance "too tenuous . . . "

The GAO report also stipulates that neither the voluntary donations nor Park Service appropriations nor Watt's official entertainment fund may be used to pay for the $1,921 breakfast hosted by Watt's wife for approximately 20 cabinet wives and other guests.

The GAO said Watt may apply the $4,500 that remains in his official entertainment fund to the cocktail party expenses, though not to pay for the breakfast. This is because unlike "the Christmas party, which was attended by government officials and their guests . . . the breakfast . . . was hosted and attended entirely by private persons," the GAO's report said.