Yugoslav President Tito's doctors said yesterday that the condition of his left leg, which was unsuccessfully operated on for a blocked artery six days ago, is "progressively deteriorating."

A tense medical bulletin said there was no change in the condition of the 87-year-old leader and implied that deterioration of vital tissues was continuing.

Reuter quoted Yugoslav sources las night as saying that gangrene has set in and that its spread could only be arrested by amputation. Tito was earlier reported to have rejected amputation because he wanted to remain a "whole man." In addition, doctors fear that Tito's diabetes and the strain on his heart from the arterial surgery would threaten the success of another operation.

While officials statements seemed designed to avoid internal panic, the Yugoslav Foreign Ministry issued a warning yesterday against any foreign interference in Yugoslavia's internal affairs, rejecting "any or all bloc contentions or rivalries concerning either its internal or international status."

But the government spokesman, Mirko Kalezic, denounced Moscow's most loyal ally, Bulgaria, over a recent article in the official Bulgarian press. The article, he said, "represents an open expression of territorial claims toward Yugoslavia and a serious threat to stability in the Balkans."

The State Department yesterday again reaffirmed U.S. support "for the unity, independence and territorial integrity of Yugoslavia." Spokesman Hodding Carter said there was "no evidence" suggesting a Soviet intervention.

He expressed confidence "that preparation for the post-Tito era -- when it comes -- will work smoothly." The spokesman also described as "apparently-routine" the current Soviet military exercises in Eastern Europe and the western parts of the Soviet Union.

Yugoslavia's armed forces have been on alert and the Communist Party organization of the armed forces yesterday vowed in a cable to Tito that the military was determined "to steadfasly and decisively defend" the independence and integrity of the country.

Although Soviet President Leonid Brezhnev has sent a get-well telegram to Tito, Soviet media so far have said nothing about the Yugoslav leader's illness. A Tass news agency dispatch distributed to foreign readers only had bitterly assailed as "fantastic concoctions" West European reports that a Soviet attack on Yugoslavia was imminent.

Ever since Tito's 1948 break with Stalin, Soviet-Yugoslav relations have been marked by chronic ups and downs.

East European sources in Moscow suggested that the Russians may be overextended as a result of their involvement in Afghanistan to undertake a direct military action in the Balkans.

"It is one thing to invade Afghanistan," one East European source said, "but quite another (to get involved) in the middle of Europe. They know it and they won't" do it.

A leading Yugoslav radion commentator, Milika Sundic, in a broadcast monitored here, said yesterday that Yugoslavia's security "is an integral part of European and world security. It must be clear to everyone that there cannot be peace in Europe if there is no peace in southern Europe and the Mediterranean area."