It was a brief explosion quickly muffled in the vastness of central Zaire. But a one-day uprising in Western Kasai province in March illustrated both the instability that lurks below the surface of political life here and the risks for those opposing Zaire's authoritarian government.

The official version is that three former legislators fomented unspecified "disorders" on March 5 in the town of Mupompa. The three, who for several years have been members of a group trying to form a second political party in this one-party state, had been consigned to the remote rural area under a special form of punishment known as "relegation," a combination of internal exile and modified house arrest.

The three were taken into custody. There were a number of persons injured, but who and how many was never specified, and political tracts "of a nature to disturb the public order" were seized. The regional governor, only recently appointed, was recalled to Kinshasa and replaced because, it was said, he had been negligent in alerting the authorities here.

Several unofficial versions floated around this capital, none of which could be confirmed. But knowledgeable sources did confirm that relatives of two of the legislators spent at least a week in Kinshasa in a fruitless search to find out where the two men were being held. Eventually, according to the State Department, the men were removed to new areas and their families allowed to rejoin them.

The incident is one of many indications that, two decades after he seized power in a 1965 coup, President Mobutu Sese Seko still governs Zaire with a tight and repressive grip.

There are currently 110 known political prisoners here, according to diplomatic and human rights sources, most of whom suspect that dozens of others are being held secretly. The State Department says its own "best estimate" is about 50.

At least 15 persons have been "relegated" to remote rural areas, according to a report issued in February by Amnesty International, the London-based human rights organization.

Zaire's secret police, known as the National Documentation Agency, routinely harass real or imagined political opponents, make illegal arrests without charge or trial and brutalize those in their custody, according to these sources.

"Virtually every Zairian taken into custody on suspicion of opposing the government suffers mistreatment or torture," stated another Amnesty report in January. "The regimens of political detainees often include deprivation of food and sleep, beatings, infliction of electrical shocks and administration of debilitating drugs."

The Reagan administration, which considers the Mobutu government one of its best friends in Africa, contends conditions have improved here in recent years. The State Department's latest human rights report notes the government during the last year has allowed the International Committee of the Red Cross to resume visits to Zairian prisons. Diplomats here say Mobutu recently appointed a high-level commission to examine the state of Zaire's underpaid and corruption-ridden judiciary.

U.S. Ambassador Brandon Grove Jr. says there have been "significant improvements over the past two years."

Government officials have heatedly denied Amnesty's torture charges, and Mobutu in his reinauguration speech last December accused foreign journalists of mounting "campaigns of disinformation and intoxication" against his government's human rights record. He said democracy and liberty were not limited in Zaire but added that they must be "reconciled with the necessities of order and discipline."