Opposition leader Joshua Nkomo, who spent two decades in prison or exile fighting white rule, fled black-governed Zimbabwe yesterday for neighboring Botswana, the Botswana government announced today.

The government said Nkomo had arrived yesterday and would remain "temporarily" but was not seeking political asylum.

The terse announcement gave no details on how Nkomo, 65, who had been in hiding since Saturday after escalating government attacks against him and his party, escaped across the border. Bulawayo, his home city, is only about 75 miles west of the frontier.

A Zimbabwean government official said Nkomo left the country "unconventionally."

The pressured departure of Nkomo, the creator of the national movement in what was then white-ruled Rhodesia in the late l940s, seems certain to end his long political career within the country. It is possible that he will mount a campaign against the government of Prime Minister Robert Mugabe, his former guerrilla ally, from outside Zimbabwe, but not from Botswana.

Despite Nkomo's hold on his minority Ndebele tribe, there was no immediate sign in Bulawayo, his political base, of any violent reaction to his departure.

The Botswana government statement issued in Gaborone, the capital, said Nkomo "intends to remain in Botswana temporarily while he explores possible ways of assisting to resolve the situation in his country.

"While he is here," the government said, "he does not intend to speak to the press," an indication of the potential embarrassment for Botswana of playing host to the nemesis of its far more powerful neighbor.

Zimbabwean refugees have been flowing into the sparsely populated southern African nation at the rate of about 40 a week this year in the face of a military offensive against dissidents allegedly loyal to Nkomo. Zimbabwe has accused its neighbor of harboring the dissidents.

In the last six weeks, Nkomo has been under increasing attacks by the government, which has charged him and his party with seeking support from white-ruled South Africa to overthrow the regime. He was detained for eight hours last month when attempting to leave the country, his passport was confiscated and he was ordered to notify police of his movements.

Independent sources say more than l,000 civilians have been killed in the last six weeks during the Army offensive. Nkomo says many of his party members have died. The government has denied the charges.

The bulky Nkomo, who weighs about 300 pounds, fled his house Saturday just before soldiers searched it and shot dead his driver. The next day Nkomo charged that Mugabe, who is currently in New Delhi attending the summit of the Nonaligned Movement, had ordered the Army to kill him. The government denied the accusation. With Nkomo seemingly bound to go into eventual exile, the future of his party, the Zimbabwe African People's Union (ZAPU) was thrown into doubt.

The party's vice-president, Josiah Chinamano, said in Harare, the capital, that he believed Nkomo was "forced to flee" by the events of last Saturday when his house was ransacked and his driver shot. Chinamano said, "The party leader's departure was a big surprise to me."

Nkomo apparently designed an elaborate ruse to cover his flight.

After being in seclusion for several days, he sent word to foreign correspondents that he would give a press conference yesterday afternoon.

No location was given, just like on Sunday night when he had a dramatic midnight press conference at a secret site. Reporters were simply told to wait to be taken to the meeting place. They, in turn, noticed that government intelligence agents were watching them as they waited in their hotel, apparently as a way for the authorities to locate Nkomo. Instead, while the reporters and agents waited for hours, Nkomo apparently slipped out of the country.