"Robert Mugabe can't tell me what to do," thundered opposition leader Joshua Nkomo. The anger welling up in his 300-pound frame caused Nkomo's voice to tremble as he spoke to a reporter about Zimbabwe's prime minister.

Despite Nkomo's outburst, however, the stability of this southern African country could well depend on Mugabe's ability to persuade -- or force -- Nkomo to assert authority over more than 2,000 dissidents who proclaim loyalty to the opposition leader.

In the six months since the prime minister fired his former guerrilla ally from the Cabinet and charged him with caching weapons to overthrow the government, Nkomo's Matabeleland stronghold in southwestern Zimbabwe has become a hotbed of violence by dissidents and counteraction by the military.

Nkomo has disavowed the dissidents, mainly members of his former guerrilla army, despite their demands for the release from detention of prisoners loyal to his party.

There is little sign of Mugabe and Nkomo reaching a rapprochement. Mugabe, from the majority Shona tribe, blames Nkomo and his political party for the dissident activity. In a recent speech in Matabeleland, Mugabe spoke disparagingly of Nkomo, saying his minority Ndebele tribe was "attributing to itself the role of royalty."

The two men, former comrades in arms in the struggle for what was then Rhodesia's independence from Britain, were both in Bulawayo last month. But they did not try to talk about the differences between them, their political parties and their tribes which are at the root of the violence. They have met only once in the eight months since Mugabe fired Nkomo and the talks did not result in any evident easing of tensions.

War-weary residents of this vast, sparsely populated area say conditions now often are like those of the guerrilla war that ended almost three years ago: they are beaten by both the dissidents and the Army.

Referring to frequent reports that the Army is brutalizing the rural inhabitants in its search for the dissidents, a white former army officer said, "They have lost the local people even quicker than we did."

He recalled how the Rhodesian army was often brutal with the local population and employed small planes with megaphones to warn the people to cooperate -- the same tactics that are being used now.

Nkomo and many of the white farmers in the area who identify with the Ndebele, say the Zimbabwean Army is responsible for most of the banditry, raping and killing that has swept the area. Mugabe and his officials deny the charges.

Sen. Garfield Todd, the last moderate white prime minister in this country, sharply criticized the Army in a recent speech to Parliament, saying, "There is something seriously wrong with our Army . . . . Which picture of government should the people accept? The splendid speeches and idealism of the prime minister or the conduct of the government as demonstrated by some soldiers?"

At least 57 whites have been killed or injured in the area in the last six months, the farmers say. Black casualty tolls are unknown because many incidents happen in remote areas, but the farmers say the total is bound to be higher.

The government has announced that it has detained 425 dissidents in the area. There are unconfirmed reports that some have been killed by the Army.

Nkomo charged in the interview that the Army is meting out "collective punishment" against the people in an area of about 5,000 square miles which has been placed under curfew since six foreign tourists were kidnaped 10 weeks ago. About 2,000 troops have been searching in vain for the hostages and their captors.

The minister in charge of security, Emerson Munangagwa, told Parliament last week that "the government has a complete grip on the security situation" and that "just a few misguided individuals" are responsible.

There are other estimates that more than 2,000 former guerrillas loyal to Nkomo are responsible for the dissidence. The Army, which integrated Mugabe's and Nkomo's guerrillas with the former Rhodesian troops, saw some battalions effectively dismantled as soldiers returned to the bush after the two top commanders of Nkomo's wartime army were arrested in March.

Reports of violence have dropped off in recent weeks but the area of the search for the hostages is closed to correspondents, making independent reports difficult to obtain.

Transportation has also been curtailed in the area. Most embassies have advised their nationals not to drive on the highway from Bulawayo to Victoria Falls. Buses along the route are held up so frequently, according to Bulawayo residents, that drivers often simply toss the gunmen their cash bag and the passengers are left alone. Tourism at the falls has diminished; one hotel has laid off 40 percent of its employes.

Despite fears when Nkomo was dismissed of a tribal war, analysts now say that is unlikely because most Zimbabweans are tired of war. The dissidents are said to have neither the firepower nor the numbers to do more than destabilize.

If Mugabe carries out plans to form a one-party state and does it without Nkomo's cooperation, however, the result could be a disaffected minority -- on the sensitive border with South Africa.

Mugabe accuses South Africa of seeking to destabilize his country. Pretoria denies the charge but dissidents in this region could provide a tempting target, similar to the rebel forces of Jonas Savimbi that South Africa champions in Angola.

Both Nkomo and Mugabe disavow tribalism even though the country voted along tribal lines in 1980, giving Mugabe a landslide victory. Nkomo discounts that election, charging that his opponents intimidated voters. But there is every indication that the next election, scheduled for 1985, will come out the same way.

In Matabeleland, Nkomo is treated like a king. In much of the rest of the country he is regarded as a disgraced former minister, hardly the fate he envisaged during three decades of fighting white rule.

"I fought 33 years in the struggle against imperialism. I suffered a lot, but in the last six months of Robert Mugabe I've suffered worse," he said. "I was badly handled. I was insulted . . . . I'm one of those who played a major role in the freedom of this country."