Patriotic Front guerrilla leader Joshua Nkomo today laid down tough conditions for his assent to a Commonwealth plan for settlement of the Zimbabwe-Rhodesia dispute, vowing to fight on until "effective white control" of the country is ended.

At the same time, he indicated his support in principle for "free and fair elections" and a new constitution for Zimbabwe-Rhodesia - the two main points of the plan drawn up early this week by Commonwealth leaders meeting in the Zambian capital, Lusaka.

Nkomo, interviewed during a four-day visit to Egypt, made his most detailed criticism yet of the Common-wealth plan that was announced Aug. 5.

Nkomo demanded that Zimbabwe-Rhodesia's white-led Army be disbanded before any elections are held, and replaced by Patriotic Front forces. He rejected the Commonwealth's plan for the elections to be "properly supervised under British government authority." He indicated that he would not attend an all-parties conference on Zimbabwe-Rhodesia, which the British propose be held in London, perhaps next month.

The conditions voiced by Nkomo, head of the Zimbabwe African People's Union, one of the two wings of the Patriotic Front, echoed demands made by his fellow guerrilla leader Robert Mugabe, head of the Mozambique-based Zimbabwe African National Union, also said the Patriotic Front guerrillas must replace Zimbawe-Rhodesian troops as the national army. He also called for the removal of Bishop Abel Muzorewa as prime minister.

Muzorewa has called the Commonwealth plan "totally unfair and in fact an insult."

Nkomo said he had spent 30 years trying to achieve free and fair elections under universal suffrage, and he would not prolong needlessly "the misery and human carnage" of his four-year bush war.

But he maintained that the armed struggle had proved the only way of achieving change.

"The Commonwealth and Britain have taken their new decision only because of the armed struggle. But for this, nothing would have stopped Britain from recognizing the Salisbury regime long ago."

It would be "very stupid" to call a halt to the guerrilla war until "the current power structure is totally dismantled," or until irreversible machinery is set up for the complete transfer of control to the blacks, Nkomo said.

By accepting the need for fresh elections and a new constitution, Britain had at last recognized the existing realities of the present Salisbury government under which the Army, police, air force, civil service "and, of course, the economy" remain in white hands, he said.

"How do you have free elections while the existing structures still remain," Nkomo asked. "The Army of change must take over and the Army of status quo must go."

The question of who would be in control at the time of the elections was skirted by the Commonwealth plan, as was the status of the existing government institutions. Observers see some possibility for compromise between the Commonwealth's plan for outside supervision of the election, and Nkomo's call for an "irreversible machinery" for transferring power.

Nkomo sharply disagreed with claims in the Commonwealth agreement, enthusistically repeated by British Prime Minister Margaret Thateher, that a major objective was to "bring about a cessation of hostilities and an end to sanctions."

"You must put things in the right order," he said. The immediate need was to improve the causes of war - effective white control and "the oppression and exploitation" of blacks, he said. Only then would the war end, he said.

In rejecting British supervision of an election, Nkomo charged Britain was "biased and prejudiced," inclined toward the existing government, and in the past always thwarted moves toward full black rule in Zimbabwe-Rhodesia.

"We are in this mess because of Britain," Nkomo declared, claiming that Thatcher's Conservative government would "distort" any agreement reached to suit her own ends.