Britain today presented a final plan for Zimbabwe-Rhodesia's transition to legal independence under black-majority rule that includes significant new assurances to the Patriotic Front guerrillas that their interests would be protected.

Lord Carrington, Britain's foreign secretary, presented the plan to Patriotic Front leaders and the present Salisbury government of Prime Minister Abel Muzorewa at the two-month-old Rhodesian settlement conference here.

The British are seeking acceptance of the plan by early next week so they can move quickly to the third and last phase: negotiating detailed arrangements for a cease-fire in the continuing war between the guerrillas and the Salisbury government. Carrington has speeded up the conference considerably this week to try to achieve final agreement before Nov. 15.

That is when Britain's economic sanctions order expires unless renewed by Parliament. Rhodesia's former white-minority government illegally declared its independence from Britain in 1965. Muzorewa's biracial government failed to win British or international recognition because the 3 percent white minority retained control of its key institutions, including the civil service, police and military.

The plan, to ensure "free and fair" elections under supervision of a British governor after a cease-fire, provides that the cease-fire and the election would be monitored by international observers and by advisory commissions of officials and military commanders from both sides.

It also provides for the release, under British supervision, of all political prisoners by both sides, and the movement back into Zambabwe-Rhodesia -- with British assistance, in time for the election -- of guerrillas, their relatives and other refugees who have been living in neighboring African nations.

"I believe these are fair proposals that should be accepted by both sides," said Carrington, the conference chairman. "They offer the only real prospect of ending the war."

He asked both delegations to give him "an early response" to the plan. He did not set a deadline for acceptance, as he did for the independence constitution at the end of the first phase of the conference, but he scheduled the next session for Monday.

The key to the success of the plan will be the response of the Patriotic Front, whose leaders, Robert Mugabe and Joshua Nkomo, have scheduled a press conference here tomorrow.

Carrington said today that he still hoped to have an agreement in time for the British governor to be on his way to Salisbury by Nov. 15. This would legally restore Zimbabwe-Rhodesia's British colonial status and permit the sanctions to be ended, under British government policy.

He left the door open to the possibility of temporarily extending the sanctions order if the conference was still continuing and making progress on Nov. 15. Informed British sources say that Prime Minister Margaret Thatcher is ready, if necessary, to push a temporary sanctions renewal through Parliament over the objections of some right-wing members of her Conservative Party.

Carrington refused to speculate today on what would happen if final agreement were not reached between both sides and the British. Other sources have indicated, however, that the British would conclude "a second-class" agreement with Muzorewa if the Patriotic Front walked out.

"I'm determined to get an agreement if it is humanly possible,' Carrington told reporters today after presenting the final transition plan at the conference. "I'm not prepared to contemplate what the British government would do if the conference breaks down."

Muzorewa already has accepted the principles of the British transition plan in an earlier draft form, so his delegation is expected to approve the final version.

But Mugabe and Nkomo had complained before today's version of the plan was presented that the British proposals were biased against the Patriotic Front.

They pointed out that the British governor would be conducting the election and maintaining law and order with the Salisbury government's civil service and police force, the Patriotic Front's opponents in the war. The guerrilla leaders also wanted their armies to have equal status with the Salisbury government's military, and be integrated with it in order to prevent its armed intercession on Muzorewa's behalf during or immediately after the election.

The Patriotic Front's concerns, expressed by Mugabe and Nkomo during this week's conference sessions, have been echoed by leaders of the black African nations that border Zimbabwe-Rhodesia, as well as by some other members of the British Commonwealth.

While the British stood firm against the Patriotic Front's main demands for integrating the opposing armies or having the cease-fire and election supervised by a United Nations peace-keeping force, they made significant additions to their original plan in an attempt to reassure the guerrillas.

These include:

Giving the commanders of the opposing armies equal status by making them equally responsible to the British governor for maintaining the cease-fire and remaining in their present positions inside the country.

Establishing "machinery on which the military commanders of both sides will be represented, to ensure compliance with the terms of the cease-fire." This machinery, to be the subject of negotiation in the next phase of the conference, probably would be a "cease-fire commission" advising the governor and would be similar to an election commission of representatives of all parties that will advise the governor on concluding the election.

Providing for an international group of individuals -- probably drawn from British Commonwealth nations, but not becoming an official Commonwealth group -- to monitor the cease-fire and the maintenance of law and order to ensure that neither side's security forces interfere with the other's or intimidate voters.

This new concept offered by the British for discussion in the next phase of the conference is still vague. These monitors are likely to be independent of the Commonwealth observers who would watch the conduct of the election across the country.

Carrington told reporters today that these potential cease-fire monitors would not be armed, nor would they be a Commonwealth version of the U.N. peacekeeping force the Patriotic Front had sought.

'Their role might be greater than observers, but short of soldiers," one British source said later.

Other elements of the plan include:

British help in organizing and carrying out the massive movement of Patriotic Front guerrillas and other refugees back into Zimbabwe-Rhodesia from neighboring countries. "A start should be made in enabling the refugees to return to their homes as soon as possible," the plan states, "and the British government will be ready to assist with the process."

Releasing all persons detained arbitrarily and on political grounds" by both sides in the war, with the British governor as the final authority for distinguishing between political prisoners and properly convicted criminals.

Extending the transition period itself from a flat two months to two months from the effective date of the cease-fire, which would be negotiated in the next phase of the conference. The British want the cease-fire to take effect within a few weeks after a peace agreement is reached.

The Patriotic Front wanted the transition period to total six months, to ensure that all its potential voters are back in the country and all preparations made for the election. The British warn in the plan that "the longer the interim period lasts before the people are given a chance to decide their political future for themselves, the greater risk of a breakdown of the cease-fire."

Providing "special arrangements," possibly British Special Branch bodyguards, "to ensure the protection of the political leaders in this period." b

Patriotic Front leaders were not prepared immediately to react to Britain's "final" offer. But spokesmen for both Mugabe and Nkomo reminded reporters that their central demand is for the guerrillas' security forces to be integrated with those of the Salisbury government, or else that an outside force be brought into Zimbabwe-Rhodesia to police the cease-fire.

Sources familiar with the deliberations of the two leaders predicted, however, that the Patriotic Front eventually would agree to move on to the next phase of the conference after reiterating its objections to the plan.

In Zimbabwe-Rhodesia, meanwhile, Reuter reported that a bomb apparently planted by guerrillas exploded in a shop in the border town of Umtali, injuring 12 persons.

In another attack about 40 miles north of Umtali Thursday night, guerrillas fired at the Montclair Casino Hotel, wounding two persons.