UNITED NATIONS, DEC. 2 -- The debt-ridden United Nations unveiled an unprecedented proposal today to borrow $50 million on the open market to remain in operation.

The idea was raised by Secretary General Javier Perez de Cuellar, who did not say what the United Nations would use as collateral. The world body's most visible asset is its headquarters, on 12 acres of prime real estate in Manhattan.

U.N. spokesman Francois Giuliani said he believed it was the first time the organization had proposed to go to commercial lenders.

In a written report to the General Assembly, which would have to authorize the borrowing, Perez de Cuellar said all U.N. reserves were exhausted and that there was a prospect of entering 1988 with barely enough cash in hand to cover two weeks' commitments.

On Nov. 16, he said, members owed $279.2 million on 1987 assessments. Total cumulative unpaid dues amounted to $456.4 million, of which the United States owed $342.8 million.

The secretary general said he had been advised that $90 million in American arrears would be paid shortly.

That payment is opposed by some members of Congress, including presidential aspirant Sen. Bob Dole (R-Kan.), who called the United Nations an "international welfare program."

Perez de Cuellar proposed in his report to increase the U.N. working capital fund to $200 million and issue interest-free certificates of indebtedness for the full amount of the arrears.

The proposed borrowing of $50 million would be at regular commercial rates.

"The United Nations must enjoy financial stability for it to serve effectively the social, economic and political needs of the world of today and of the next century," Perez de Cuellar said.