President-elect Tancredo Neves underwent "critical" surgery today for the fifth time as doctors fought against the rapid spread of bacterial infection. As the 75-year-old political leader struggled for his life, questions arose about circumstances surrounding the first operation in Brasilia March 15.

Presidential spokesman Antonio Britto said that during today's two-hour operation, doctors at Sao Paulo's general hospital under head surgeon Henrique Walter Pinotti made two small incisions to remove pockets of infection.

These centers of infections had caused a fever overnight and were detected by special tests this morning. Neves has been receiving massive doses of antibiotics, some flown in especially from the United States. He has also suffered from lung problems and is connected to an artificial breathing machine.

Today's operation sparked reports that Neves had died and that troops had been placed on alert here. Spokesman Britto denied the reports but acknowledged that Neves had experienced serious difficulties after the operation.

"At this moment, the situation is stable, but it is both delicate and critical," Britto said. He said that Neves was in the recovery room with his wife and family and spoke of a "healthy and justifiable optimism."

The commander of armed forces in the San Paulo region also issued a statement denying that his troops had been placed on alert.

The latest surgery followed removal of infected material on Tuesday and confirmation that he was suffering from a highly resistant form of Pseudomonas bacterial infection contracted in Brasilia.

Seven-and-a-half hours after the operation, Britto said that Neves was "reacting favorably and the situation was encouraging" despite his having suffered a cardiovascular crisis soon after leaving the operating room.

However, Britto said Neves continues to depend on a respirator. A surgeon from the hospital said that without it, he would not survive. The surgeon said a telephone line was being kept permanently open to medical experts at Harvard University who were advising on treatment of the 15 different types of bacterial infection.

Political leaders admitted that the situation was "very grave." Sao Paulo State Gov. Franco Montoro said, "The hopes of the nation are that the resistance he has shown up to now will guarantee him strength."

Neves' election after 21 years of military rule had raised hopes for a democratic revival that would unite this huge nation burdened with the world's biggest external debt. But his inability to take office has caused a loss of momentum for the new administration's policies of transition and a rapid shift of power to the Congress.

Neves first underwent surgery for diverticulitis on the eve of his scheduled inauguration. Later, two operations were necessary to correct burst sutures and a trapped intestine. The fourth and fifth operations were to remove centers of infection.

These infections first began at Brasilia's military hospital, and a well-informed government official confirmed that on March 15, "during the climate of tumult, the rules of the hospital were broken."

The official, who requested that his name not be used, said, "The number of people who entered the operating room was very high, though I don't know of any nonmedical personnel."

A Sao Paulo newspaper had charged that several officials, including Communications Minister Antonio Carlos Maghalhaes and a senior Foreign Ministry figure, had been present for the surgery. This was strongly denied.

Acting President Jose Sarney, 54, instructed those ministers still in Brasilia to remain at their desks. The government leader in the lower house of Congress and "third man" of the new administration, Ulysses Guimaraes, voiced confidence that the situation was under control.