Yugoslav doctors fighting to save the life of President Tito today for the first time officially described his condition as "critical."

The country's armed forces were reported to have been put on precautionary alert and some reservists called up.

Further indications of the seriousness of the 87-year-old Yugoslav leader's condition were provided by the return of the prime minister, Veselin Diuranovic, from a visit to East Germany a day earlier than expected. Meanwhile the foreign minister, Josip Vrhovec, canceled a tour that he was due to make to Asia.

The latest bulletin, issued by Tito's eight-man medical council read: "In the course of last night, the condition of health of President Josip Broz Tito was critical. After intensive measures of treatment had been taken there was a certain improvement but his conditions remains very grave. Appropriate medical measures are continuing."

The president's two sons, Marko and Misa, were understood to be standing by at the clinic in Ljubljana, in northwest Yugoslvia, where he is being treated. It is not known whether his estranged wife Jovanka, who disappeared from public view in 1977 after 25 years of marriage to Tito, was also summoned to the hospital.

The 260,000-strong Yugoslav armed forces were reported on a preliminary state of alert with leave canceled. Many of Yugoslavia's half a million reservists were also called up to take part in exercises.

One private baker in Belgrade complained to his customers that he had lost four out of five members of his staff in the last three days. All were summoned for Army service as Tito's condition deteriorated.

Government officials, journalists, factory directors, and other key personnel were also told to remain in close touch with their offices and cancel any holiday plans.

Outwardly, the atmosphere in Belgrade and other major Yugoslav cities appears remarkably calm but ordinary people are hurrying back home in the evening for regular TV bulletins on the president's health. Press coverage of Tito's illness has been restricted to publishing the medical bulletins -- a marked contrast to the large amount of space being devoted to the Winter Olympics in Lake Placid.

Tito's month-long illness took a dramatic turn for the worse last weekend when it was announced that his heart and kidneys had weakened. Unitl then, it had been assumed that he was making a remarkable recovery from the amputation of his left leg on Jan. 20.

State television tonight gave prominence to a statement by President Carter at a press conference last night stressing that Yugoslavia is a stable country well able to defend itself. Analysts said the attention given to the president's statement reflected official Yugoslav approval of the U.S. adminstration's diplomatic handling of the sensitive issue of Tito's illness.