Doctors attending President Tito tonight issued their most pessimistic-sounding bulletin yet on the health of the 87-year old Yugoslav leader, admitting for the first time that he has heart problems.

Medical sources said it now appears certain that Tito's health has undergone a major deterioration following amputation of his left leg three weeks ago. A bulletin issued Sunday spoke of kidney and digestive problems, and the former World War II guerrilla leader also is known to be suffering from diabetes and arteriosclerosis.

The latest bulletin issued by the eight-man medical council, all of them Yugoslav professors of medicine, read: "In the treatment of President Josip Broz Tito, further difficulties in connections with the functioning of the kidneys are present. The treatment has been made more difficult because of certain signs of heart weakness. Necessary medical measures are being undertaken."

The worsening in Tito's condition coincided with a strongly worded statement by his top of foreign policy adviser, Milos Minic, stressing that this country is well able to look after is own defense. The statement in an interview with Yugoslav journalists was interpreted here was a warning to the Soviet Union not to interfere in Yugoslavia's internal affairs should Tito die.

Minic, a member of the 24-man Communist Party Presidency, Yugoslavia's highest policy making body, warned of the danger of a return to "the laws of jungle" in international relations following Soviet intervention in Afghanistan. Officials have not hidden their concern that a breakdown in detente could have serious implications for Yugoslavia's own independence.

Tito was admitted to a hospital in the northwestern Yugoslav city of Ljubljana a month ago. On Jan. 12 surgeons failed in an attempt to bypass an arterial blockage in his leg with an artificial duct. The leg later had to be amputated.

Despite earlier optimistic forecasts of his rapid recovery, it is evident that the two operations eight days apart proved a severe blow to his condition. No pictures of him have been published for more than two weeks and reliable reports on his condition have been scant.

Tens of thousands of messages have been ariving at the Ljubljana hospital wishing Tito a speedy recovery. In one such telegram that authorities said was typical, a Serbian schoolboy wrote:

"I am five years old and I love you a Lot. Don't ever be sich again."

Marshall Tito led a Yugoslav partisan uprising against invading German forces in World War II, and for most of this country's 22 million inhabitants he is the only head of state they have ever known. Under his leadership, Yugoslavia became the first communist satellite to break away from the Soviet Bloc.

" the important thing is that Tito lives," said one government figure, expressing a sentiment that has been repeated here in recent weeks. "As long as he's alive, the Russians won't dare touch us."

The flurry of concern over Tito's health last month has helped prepared ordinary Yugoslavs for the possibility of his death, and there are now fewer signs of tension. A state of alert, including the mobilization of reserve Army units, has been relaxed.

Nevertheless, the death of the man who has ruled Yugoslavia for 35 years would be an immense psychological blow to a nation that generally views him in the grand tradition of Balkan patriarchs: as a father figure wo protects his people from a hostiled, outside world.

In the event of Tito's death a collective leadership composed of representatives of each of Yugoslavia's different nationalities would assume responsibility for running the country. Political anaalysts believe the short-term transition to a post-Tito era should proceed farily smoothly.

In the longer term, Tito's successors would have to act skillfully to avoid a reemergence of deep-rooted grievances, fueled by the economic gulf between the developed north and underdeveloped south of the country.