A top guerrilla leader today laid down conditions for a settlement of the Zimbabwe-Rhodesian issue that have long been unacceptable to the leadership of the white minority in that war-torn country.

South African Foreign Minister Pik Botha, meanwhile, said his government was "deeply disturbed" over the plan proposed yesterday by Britain and the Commonwealth to devise a government for Zimbabwe-Rhodesia capable of achieving international recognition.

Botha said at a meeting of South Africa's ruling National Party Monday night that events had "suddenly, virtually overnight, taken a very serious course." South Africa is the key outside supporter of the embattled Zimbabwe-Rhodesia government of Prime Minister Abel Muzorewa, who earlier called the plan for new elections "unfair" and an "insult to the electorate."

Thus, less than 48 hours after it was unveiled, the Commonwealth plan to resolve the 15-year-old Rhodesian independence issue continued to run into difficulties with the parties most closely involved.

Long-time observers of the seemingly intractable situation noted, however, that first reactions in such diplomacy can often be discounted as public posturing to establish a maximum position.

Both Tanzanian President Julius Nyerere and British Prime Minister Margaret Thatcher, the key architects of the agreement, have spoken cautiously about a step-by-step approach to a peace settlement. In separate press conferences today they refused to be drawn out on contentious issues still to be dealt with.

Robert Mugabe, head of the Zimbabwe African National Union, one of the two wings of the guerrilla forces, laid out three prerequisites to a settlement involving just such issues.

He said the constitution must lead to the removal of Muzorewa and the disbanding of the white-controlled Army, Air Force and police. He added that the Patriotic Front guerrilla forces must be accepted as the national Army.

Numerous negotiations with the white-minority government of former prime minister Ian Smith bogged down over such problems.

In addition, a spokeman here for Mugabe's faction held a contentious press conference in which he said the guerrilla group would not be in favor of sitting at the same conference table with Muzorewa. Edgar Tekere, secretary general of the faction, repeatedly referred to Muzorewa, elected in controversial elections last April, as a traitor.

His remarks followed a moderate opening statement that gave an optimistic appraisal of the Commonwealth agreement.

Tekere said the Commonwealth plan "encompasses the basic principles" of the Patriotic Front and did not contradict its position. He added that he hoped "the British government will remain faithful to the spirit of the declaration."

Under questioning, however, he came out against Muzorewa's participation and said the Front would not commit itself to attending a conference until Britain announced the agenda.

It was felt that some of the tough talk was aimed at maintaining guerrilla morale in the field.

The other wing of the Patriotic Front, the Zimbabwe African Peoples Union headed by Joshua Nkomo, was not represented at the press conference. Nkomo is in Ethiopia on a visit.

Earlier, Nyerere, the chairman of the front-line states supporting the guerrillas, said he was confident that the Patriotic Front would attend the conference called by the British and said he felt that the problems would come from Smith and Muzorewa, supported by South Africa.