Joshua Nkomo, a founding patriarch of Zimbabwe, accused his longtime rival, Prime Minister Robert Mugabe, today of twisting the promise of Africa's newest country into "a reality of suspicion, terror and failure."

Spurning recent government efforts to ease political tensions here, Nkomo rallied his opposition party with a bitter and blistering speech that cited what he called "the unspeakable crimes" against civilians committed by Army troops over the past two years.

These crimes, he charged, were committed during a campaign against armed dissidents in southern Matabeleland, Nkomo's political stronghold. "What are we to say about the slaughter of children, the raping of mothers and sisters, the torture of old men and women?" Nkomo asked his followers.

"We say, enough. We accuse the perpetrators of this crime before all the world . . . . We appeal to all decent, humane and humble folk, to the people of Zimbabwe, to act now and stop this insanity before it destroys us all," Nkomo said.

Nkomo's speech was a call to his followers to embark upon what may prove to be the 67-year-old leader's last political campaign.

Nkomo was one of the earliest leaders of the black nationalist struggle against white-minority rule here. Many of his supporters wore badges today honoring him as "Father Zimbabwe."

The speech was delivered on the opening day of the first party congress Nkomo's Zimbabwe African Political Union (ZAPU) has held since the country gained independence in 1980.

Mugabe has pledged that if his party wins parliamentary elections planned for early next year he will move the country toward a one-party state that will effectively mean Nkomo's political extinction.

But this two-day party congress, held two months after Mugabe's Zimbabwe African National Union (ZANU) conducted its own gathering here, is one of several indications that Nkomo and his followers are not yet finished.

The meeting has drawn a respectable crowd of 5,000 to 6,000 supporters to this capital city, which is in the heart of Mugabe's political constituency. It follows by four days a small but significant political triumph in Bulawayo, the country's second largest city and unofficial capital of Matabeleland, in which Nkomo's party swept all of the seats in municipal elections.

Despite this, most analysts believe Nkomo will be hard-pressed to hold onto the 20 seats his party won in the 100-member Parliament during the first national election in 1980. Mugabe's party won 57 seats in balloting that divided the country largely along ethnic lines. Nkomo won support from the Ndebele-speaking minority of the south, while Mugabe won the Shona-speaking majority of the northern and central provinces.

At first the two men, who headed rival guerrilla movements during the war for independence, joined forces in the government, with Nkomo heading the Ministry of Home Affairs. But in 1982, Mugabe charged Nkomo was seeking his overthrow and ousted the ZAPU leader from the Cabinet.

The subsequent security crackdown by the government has resulted in the deaths or imprisonment of an unknown number of local ZAPU leaders and has persuaded thousands of Matabeleland residents to purchase ZANU membership cards for personal safety. Whether they will choose to defy the government and vote for Nkomo's party in next year's election remains an open question.

In an interview before the speech, Nkomo said reconciliation with Mugabe was still possible.

While the conclusion of Nkomo's speech called for a "united front" between the two parties, he appeared far more intent on settling old scores and baiting his political rivals than on preaching unity.

"Their hands are stained with the blood of the people," Nkomo said. "No genuine revolution spills the blood of the people or recruits members with a bayonet. If the ZANU leadership intends to behave like fascists, we cannot call them anything but fascists."