President-elect Tancredo Neves underwent intestinal surgery in Brasilia today for the second time in a week as concerns mounted over the health of Brazil's first civilian leader in 21 years.

Neves, 75, who was rushed to surgery last Thursday on the eve of his scheduled inauguration, was operated on for several hours this afternoon at Brasilia's Hospital de Base, officials said. This evening, presidential spokesman Antonio Britto said the operation was "very successful" but offered no other information on the condition of the president-elect.

Neves, who was first operated on for diverticulitis, an inflammation of the intestine, was reported by associates today to be suffering from complications that prevented his intestines from resuming their normal function. Carlos Mosconi, a congressman in contact with Neves' family, said this afternoon that the operation was intended to correct an intestinal blockage and had been completed "simply, rapidly and without difficulty."

Vice President Jose Sarney, who was elected with Neves Jan. 15 and inaugurated on schedule Friday, has been serving as acting president during the last six days. Officials here said Brazil's new government has been virtually paralyzed by the decision of Sarney and other officials not to implement policies or make appointments without Neves' approval.

The second operation followed several days of indications that Neves was not recovering adequately from his first operation. Although he was scheduled to move to a suite in the hospital Saturday, Neves remained in the intensive care unit. He has not been photographed nor made a public statement and has not received vistors other than his immediate family.

On Sunday, Neves' doctor said some congestion had developed in the president-elect's lungs, although the condition was later described as minor. Yesterday, a team of nine doctors assembled in Brasilia by government leaders and Neves' family reported that there were "alterations in intestinal movement" that were impeding a full recovery.

Preparations were made yesterday to transport Neves to a hospital in Sao Paulo for a new operation, hospital officials said. Doctors later decided to postpone the operation in the hope of correcting the complications with clinical treatment, the officials said.

Mosconi, a physician and member of Neves' Brazilian Democratic Movement Party, said in a television broadcast this afternoon that Neves had a blockage, known as a mechanical ileus, in the small intestine. The blockage prevented the intestine from functioning and caused an accumulation of gases and swelling, he said.

Mosconi added that Neves' recovery from the new operation could take up to 30 days. Until today, some government officials had predicted that the president-elect would take office by next Monday.

Cabinet ministers and other political leaders of Neves' new administration hurried to the hospital this afternoon shortly after the surgery was announced. Until now, top government officials have publicly maintained that Neves will have no difficulty assuming office and have delayed appointments of second-level ministerial officials in anticipation of his quick recovery.

Political observers here and in the capital said that a continued deterioration of Neves' health could provoke a major political crisis in his delicately constructed governing coalition. Sarney, 54, was president of the ruling, military-backed Social Democratic Party until last July and might be opposed as a permanent president by sectors of Neves' party, these sources said.

Neves, whose career as an elected official has spanned half a century, has dominated the leadership of his Democratic Alliance coalition since it was formed by the Brazilian Democratic Movement and dissident sectors of the Social Democratic Party last July. The president's personal leadership and ability as a conciliator of diverse factions of the right and left were considered essential in the opposition's victory over a military-backed candidate in the indirect presidential election by a special electoral college.