Zaire's former prime minister today announced his formal opposition to President Mobutu Sese Seko and warned that vital economic and geopolitical interests could be compromised by continued Western backing of Mobutu's rule in the strategic central African nation.

Speaking out for the first time since he left Zaire suddenly, flew here and resigned April 16, Nguza Karl-I-Bond avoided calling for an armed uprising against Mobutu, who has ruled virtually single-handedly since seizing power in 1965. But he said he would not attempt -- as he claimed he has in the past -- to stop an uprising if one breaks out.

Nguza is known as a moderate, strongly pro-Western political figure. His call for opposition came when Mobutu's political fortunes were seemingly at a low ebb with the recent defeat of his closest European ally, former French president Valery Giscard d'Estaing.

The country's chronically troubled economy received a needed reprieve, however, with the announcement Tuesday that the International Monetary Fund will estend a $1.1 billion credit. International lending organizations are expected to continue to keep Zaire's austerity plan under close scrutiny because of Mobutu's history of economic mismanagement.

In a rambling two-hour interview, Nguza charged that Mobutu had managed to skirt World Bank and IMF safeguards and had allegedly skimmed off millions of dollars in state funds to add to his private fortune.

While holding out the slim hope that Mobutu would step down or allow meaningful reforms, Nguza said his desire for a peaceful solution had limits. He said he planned to release the text of his opposition appeal to the Zairian people in Paris Thursday.

"The West must intervene to avoid a bloody, violent upheaval which would compromise Western interests in Zaire," he said.

He said that after his resignation he had been approached by fellow Lunda tribesmen living in neighboring Zambia and Angola, who expressed willingness to invade their native province of Shaba, where major rebel invastions took place in 1977 and 1978.

"I told them to do nothing," he said, "but if I am not understood and if a popular revolt and another war breaks out, I will be behind my people and assume my responsibilities to the bitter end."

When he was foreign affairs minister in 1977, Nguza was accused by Mobutu of involvement in an insurrection, arrested, tortured and condemned to death for high treason. He was rehabilitated two years later, reinstated to his foreign ministry post and made prime minister in 1980.

Nguza stressed his ties with the West and his abhorrence of communism during the interview, which took place at the suburban estate given to him by Mobutu after his rehabilitation. But running through his remarks was the theme that the hard-pressed Zairian population now held the West responsible for keeping Mobutu in power.

"I'm pro-West and share the West's very valid concern for keeping Zaire out of communist hands," he said, "but the human rights violations, the economic misery, the president's pilfering of state coffers is equally important. And if nothing is done, then there's a risk the people will take justice into their own hands."

Nguza, 42, is the nephew of the late Moise Tshombe, the leader of the post-independence secession of mineral-rich Katanga (now Shaba) Providence, who later became prime minister of Zaire.

At the time of his resignation, Nguza was in charge of implementing the internationally backed economic and financial reforms needed to restore health to the strapped economy. He said his call for opposition by the Zairian people was justified by Mobutu's alleged financial peculations. He charged that Mobutu, by exerting personal control over the Central Bank of Zaire, the nationalized Gecomin mining company and the Sozacom mineral exporting firm, had undermined the reforms that nguza was charged with overseeing, and had skimmed off more than $100 million last year and $26 million in the first quarter of 1980.

Nguza said the skimming of Gecomin, which normally produces 63 percent of the state budget, and of Sozacom, which once provided 70 percent of Zaire's foreign exchange earnings, was responsible for the recent 40 percent devaluation of the currency.

He said suspicion of Mobutu funneling state money into his personal accounts led the IMF and World Bank to insist on stricter controls on the disbursement of the $1.1 billion of "extended facilities" over three years approved today.