Prime Minister Robert Mugabe today fired Joshua Nkomo and three members of Nkomo's Patriotic Front Party from his Cabinet, charging them with stashing arms "to overthrow my government."

In a highly charged press conference announcing the ouster, Mugabe also accused Nkomo, leader of the junior party in Zimbabwe's coalition government, of attempting to conspire with white-ruled South Africa to overthrow the government.

The prime minister said police are investigating arms caches discovered on Patriotic Front property in recent weeks, and he hinted that arrests of Nkomo and other party leaders may follow. But he left the door open for continuing his ruling coalition by retaining four other ministers and deputies from the party.

Nkomo, furious at not being told personally by Mugabe of his dismissal, denied the prime minister's charges and made it clear that the coalition was over and that he would lead the Patriotic Front in opposition. "We went in to the Cabinet as a party. We go out as a party," Nkomo told reporters.

Nkomo is the leader of the militant minority Ndebele tribe centered in southwestern Zimbabwe. It is feared that his dismissal could lead to renewed civil strife in this key southern African nation, which is often regarded in the West as a potential model for a successful multiracial society.

Many analysts see the removal of Nkomo and recent personal attacks against him as a major step in Mugabe's long-sought plan to establish a one-party state. Even without Nkomo's party, Mugabe commands a decisive majority in Zimbabwe's parliament.

Just a year ago the guerrilla armies of Mugabe and Nkomo, remnants of a seven-year war for black-majority rule, clashed for several days with about 300 deaths resulting.

Since then, however, the two guerrilla groups have been disbanded and their troops integrated into a national Army that also includes remnants of the forces that fought to maintain white rule in this nation, formerly known as Rhodesia.

Mugabe, leader of the majority Shona tribe, today appeared mindful of the potential for conflict. Although he spoke in derogatory terms about Nkomo and accused him of "dishonesty" over the arms caches, the prime minister limited his charges to the top leadership of the party, which is also known as the Zimbabwe African People's Union (ZAPU).

"I'm sure there will be people who have a big case to answer," said Mugabe, adding later, "It cannot be denied that the arms were being hoarded for the purpose of trying to overthrow my government."

But after announcing the ouster of Nkomo, two senior ZAPU officials--Transportation Minister Josiah Chinamano and Natural Resources Minister Joseph Msika--and a deputy minister, Mugabe spoke of party regulars in conciliatory terms.

"It is very clear to us that very many people in ZAPU did not know what was going on," he said. "Only a clique, perhaps at the top, plus some commanders, knew about it. We cannot be unfair to the ordinary ZAPU follower and we don't begin to accuse him of peparing for war."

He said two other ministers and two deputies from ZAPU had been told they were "free to continue in government." With the exception of "a few" of Nkomo's former military commanders, "the same goes for other appointed people in government" and the Army, Mugabe said.

Nkomo, despite his anger, was careful not to encourage violence when he met with reporters following the dismissal.

"One hopes there's no strife," Nkomo said. "It would be a tragedy." Noting the difficulty of bringing about unity after the long war, he added, "My prayer is that it will not disintegrate."

The split between Mugabe and Nkomo has antecedents in years of conflict between their separate political organizations. Their guerrilla armies clashed at times during the war against white rule, frequently causing each other more casualties than those inflicted by their mutual enemy.

Nkomo began the struggle against white-minority rule three decades ago, but his party split in 1963 and the Zimbabwe African National Union was formed. Mugabe took over leadership of the organization in 1975 after a bitter internal struggle.

The armed struggle that began in 1972 brought the two parties into a shaky alliance known as the Patriotic Front. It fell apart when Mugabe refused to run jointly with Nkomo as a unified party in the Zimbabwe's first elections in 1980.

Mugabe's party scored a landslide victory but allowed Nkomo's party to join in a coalition government in a move to prevent civil war.

That coalition having now ended, each leader had bitter words for the other.

Mugabe called the storing of arms "utter dishonesty," adding: "We feel cheated. We feel those we've trusted as partners have turned out to be despicable persons."

He also charged that Nkomo met twice with South African officials shortly after the election to see if Pretoria would support him if he attempted a coup.

Nkomo denied the charge of collusion with South Africa, accusing Mugabe of "telling a lie--a straightfoward lie." He said Mugabe's charge was a "made-up story, an excuse to get rid of me."

Nkomo's voice rose in anger over the fact that he was told about his firing by reporters, rather than by Mugabe directly. "What is he frightened of?" Nkomo asked. "Why doesn't he face me?"