Victor Paz Estenssoro, the centrist social reformer sworn in today for a fourth term as president, vowed to renegotiate Bolivia's $3.4 billion foreign debt "without taking bread from the mouths of the people."

In a half-hour speech before the Congress and diplomats from 41 countries, Paz said he would open the state-dominated economy to foreign investments and encourage joint ventures in the key sectors of oil and mining.

"The country cannot remain permanently paralyzed in poverty out of fear of letting in foreign investment," said the one-time revolutionary nationalist, who won a runoff election in Congress yesterday.

Paz, 77, also vowed to crack down on the billion-dollar illicit cocaine trade and "cut off corruption at the root."

This is the first time in more than 20 years that power has changed hands here from one constitutionally elected government to another.

Paz succeeds Hernan Siles Zuazo, a social democrat who proved unable to control the government or the economy, now registering a 30,000 percent annual inflation rate. Siles was pressured into calling elections and ending his mandate a year early. Unions that nominally backed him struck each time he tried to reduce inflation.

Paz finished second to conservative former general and president Hugo Banzer Suarez, 58, in a July 14 popular election, in which none of the 18 candidates won the required absolute majority. Paz garnered enough centrist and left-wing support to beat Banzer in the runoff.

In Bolivia's fissured politics, the coalition that Paz patched together to win could prove to be his undoing. Yet Paz, an economist and lawyer, has been a key player in virtually every political upheaval in this Andean nation for the past 40 years.

Known as a leftist reformer in his youth, his politics have been refitted often to the country's changing political times. He was born in October 1907 in Parija, a city on the Argentine border. His political career began in 1940, when he cofounded, with Siles Zuazo, the Nationalist Revolutionary Movement, which has dominated party politics here ever since.

Paz won the 1951 presidential election but was barred from taking office by the military, which feared his left-wing programs. The next year, he led a social revolution, and took power after three days of bloody clashes with the Army.

Nationalizing privately held tin mines, including those of the Patino family for whom he once worked as an attorney, Paz also distributed millions of acres of farmland and instituted universal suffrage.

Siles Zuazo won the 1956 elections, but in 1960 Paz was returned to office. He was reelected in 1964 but was overthrown three months later by his vice president, right-wing Air Force general Rene Barrientos. Paz fled to exile in Peru.

Siles' election in 1982 ended a long period of Army rule, but Paz became increasingly estranged from his longtime ally in the chaotic years that followed. Drought in the highlands and plunging prices for minerals worked against Siles. The government packed the public payroll and printed billions of pesos.

Bolivia has not paid principle or interest on its debt since March 1984, and debt service would cost the government 150 percent of its export earnings.

Paz announced that he will implement a free exchange rate for the peso. He also was expected to open negotiations with the International Monetary Fund and trim the larded public sector. He warned that the heavily subsidized government mining company will have to function on its owns profits.

Political observers here note that Paz does not have the backing in the legislature to implement tough economic measures.