Vice President Jose Sarney inaugurated Brazil's new civilian government here today as President-elect Tancredo Neves recuperated in a hospital from an emergency intestinal operation.

Sarney, elected with Neves last Jan. 15 by a special electoral college, took the oath of office as vice president in a ceremony before the congress this morning. Under Brazil's constitution, he assumed the powers of the presidency, which was vacated today with the expiration of President Joao Figueiredo's six-year term.

Neves, 75, was prevented from taking office by a constitutional requirement that he be sworn in before the congress. He was reported to be recovering satisfactorily today at Brasilia's Hospital de Base following a 1 1/2-hour emergency operation last night.

Government officials said that Neves would be inaugurated as soon as he is able to leave the hospital. His doctors estimated that he would be hospitalized for a week or more and would not be able to work for 10 to 15 days.

The president-elect was rushed to the hospital at 10 p.m. yesterday suffering from severe abdominal pain, according to official accounts. His condition, originally diagnosed as appendicitis, was later determined to be diverticulitis, a disorder of the intestine, officials said.

Neves underwent surgery early this morning to remove the inflamed tissue from his intestine and was later transferred to a private suite in the hospital. His spokesman, Antonio Britto, reported late today that Neves was resting and had undergone some physical therapy but had not yet been allowed visits with the relatives and aides maintaining a vigil at the hospital.

Sarney presided over the swearing-in of the new government's Cabinet as well as a reception this afternoon for dozens of visiting foreign dignitaries, including Vice President Bush, at the Itamaraty Palace of the Foreign Ministry. Three speeches planned for delivery by Neves were canceled, along with a ceremony transferring the presidential sash and other public events.

The inaguration marked the end of 21 years of authoritarian rule by Brazil's armed forces and the beginning of a transition to full democracy in Latin America's largest and most powerful country. Brazil has a population of 130 million and is the ninth nation in the hemisphere to move from military to civilian government since 1979.

Sarney, 54, served as a senator and state governor under military rule and was president of the military-backed Social Democratic Party from 1980 until last July. He was named as Neves' running mate after resigning from his party post and joining a group of party dissidents who allied with Neves' opposition Party of the Brazilian Democratic Movement in the Democratic Alliance.

Despite his age and sudden illness, Neves' doctors say he is in excellent overall health. Renault de Mattos, Neves' longtime personal doctor, said Wednesday that a recent examination of the president-elect had yielded the same results as one he performed 20 years ago.

Neves was reported to be ailing Wednesday and yesterday from a cold and laryngitis. However, he continued to receive visitors at his home, the Granja do Riacho Fundo, and yesterday attended mass only hours before he was taken to the hospital. Today, incoming Justice Minister Fernando Lyra said doctors had been monitoring Neves' abdominal condition for several days but had believed treatment could be postponed until after today's scheduled inauguration.

Bush, who arrived here last night, was scheduled to meet with visiting ministers of El Salvador, Costa Rica, Honduras and Guatemala Saturday and with Interim President Sarney before returning to Washington.

Nicaraguan President Daniel Ortega approached Bush this morning at Sarney's inauguration and spoke to him informally for several minutes. However, U.S. officials said no official contact between the American and Nicaraguan delegations here was planned.

According to U.S. Ambassador Diego Asencio, Ortega asked Bush, "You really want us to have peace?" The vice president replied that Nicaragua's leftist Sandinista government should restore religious and press freedoms and free elections, The Associated Press quoted Asencio as saying.

Bush then told Ortega, "The reason we are here is to honor democracy. This is what democracy is about," the ambassador reported.

Neves' hospitalization was the first disruption in what has been a remarkably smooth transition from military rule to the civilian "new republic." Military leaders, who managed to control to a large degree the terms of their withdrawal from power, have promised repeatedly to respect the new political order and made no open effort to intervene when Neves was hospitalized.

Neves, a politician first elected to office 50 years ago who is renowned for his skills as a cautious consensus-builder, named a Cabinet representing a careful balance between center-left sectors of his own party and more conservative elements of the Democratic Alliance coalition that reflect his own views.

The new president's most important appointment may have been that of his nephew, Francisco Neves Dornellas, as economy minister. Dornellas, who served as secretary of federal revenue under the outgoing Figueiredo government, is known as a relatively conservative technocrat. He is expected to be the leader of what Neves has promised will be an aggressive campaign to crush annual inflation approaching 300 percent.

Dornellas is also expected to play a key role in Brazil's effort to negotiate a new economic agreement with the International Monetary Fund and a long-term rescheduling of foreign debt payments with international banks in the coming months. Now calculated at about $100 billion, Brazil's foreign debt is the largest in the developing world.

In domestic policy, Neves' administration is expected to be occupied for up to three years with the reform of the Brazilian constitution and political institutions -- such as a relatively weak Congress and restricted labor unions -- molded along the military's authoritarian lines.

A congressional commission is studying a series of electoral reforms to be implemented this year.