The Patriotic Front guerrillas today withdrew their objections to a British-proposed constitution for an independent Zimbabwe-Rhodesia, giving the six-week-old settlement conference its first substantial negotiating success.

Guerrilla leaders Robert Mugabe and Joshua Nkomo told British Foreign Secretary Lord Carrington that there was no need for further discussion of the constitution if the conference, which also includes the rival Salisbury government, could agree on transitional arrangements leading to independence.

Since their reservations to the constitution have been dropped, the conference can move to tripartite discussions of the more difficult problems of working out interim arrangements leading to legal independence.

Britain started separate talks Tuesday on the transition with Zimbabwe-Rhodesian Prime Minister Abel Muzorewa, who has accepted the constitution. It was a move that threatened a breakdown of the conference, which is regarded as the last reasonable chance to settle the 14-year-old dispute peacefully.

Both sides were at pains not to claim that the other had given in to reach an agreement on a formula to solve the key remaining constitutional issue -- compensation for expropriation of white-owned land.

There appeared to be elements of "give" on both sides, although perhaps more from the guerrillas.

Mugabe and Nkomo told Carrington in a 15-minute meeting that "there will not be need to revert to discussion on the constitution" if they are satisfied with the transitional arrangements that are worked out.

Thus, they gave Carrington the key word he had wanted -- "will." Previously, they had been willing only to say that it "may" not be necessary to reopen constititional talks, and that had not been enough for the foreign minister.

Carrington, however, also gave some ground as the Front did not specifically say that it agreed to or accepted the constitution. On Monday, Carrington froze the Front out of the transitional talks until the guerrillas could accept the constitution "without ambiguity."

Instead, Carrington chose to interpret today's Patriotic Front statement to mean acceptance. He welcomed the statement and added that he "looks forward to beginning discussion of the transitional arrangements in plenary session."

The question of acceptance has great significance to the Front, which maintains there can be only one agreement at the end of the conference.

It fears that if the conference breaks down over the transitional issues -- still a distinct possibility -- the British and Muzorewa still might implement the constitution and hold elections in hopes of getting wide international acceptance on the ground that the Front had signed off on the charter.

The purpose of the conference, which Britain is sponsoring as the legal colonial power in Zimbabwe-Rhodesia, is to bring about agreement on a constitution and the transitional issues including elections, disposition of the warring military forces and a cease-fire.

This would then lead to international recognition of the resulting independent government and to the lifting of economic sanctions imposed as a result of the white minority's illegal declaration of independence in 1965.

The London conference is the ninth major effort to solve the problem that has boiled over into a bloody war between the Salisbury government and guerrillas in which more than 20,000 people have been killed.

It was apparent that today's break-through in the first conference meeting attended by the guerrillas in a week came as a result of pressures exerted on both the Patriotic Front and Britain from a variety of sources, mainly over the land compensation issue.

Leaders of several Commonwealth and African nations criticized Carrington's conduct of the conference and warned him that there could be no acceptable solution without the Patriotic Front.

At the same time, the five "front-line" states neighboring Zimbabwe-Rhodesia that support the guerrillas urged a moderate approach by their clients. The "front-line" states are Mozambique, Zambia, Tanzania, Botswana and Angola.

It was noteworthy that today's agreement came just one day after a "front-line" summit in Tanzania that supported the Front on the land issue but took a significantly conciliatory approach, urging that the conference "move to the next crucial stage." The guerrillas are under pressure to settle from their supporters, particularly Zambia and Mozambique which have been attacked repeatedly by Zimbabwe-Rhodesian forces.

Last week Carrington made the proposal on the land issue that the guerrillas accepted today. He offered vague promises of financial assistance in a multinational grouping to help an independent Zimbabwe government pay for land expropriated from whites as required under the British-proposed constitution.

About 6,000 white farmers control most of the best land in the country and it is estimated that it could cost more than $500 million to buy out most the farmers in order to carry out a land reform program.

When the talks broke off Monday, the Front , citing the overriding importance of the land issue, said it was seeking "clarifications" from various governments about contributions to a land compensation plan.

Today, the guerrillas said in their statement, "We have now obtained assurances that depending on a successful outcome of the conference, Britain, the United States and other countries will participate in a multinational financial donor effort to assist in land, agricultural and economic development programs.

"These assurances go a long way in allaying the great concern we have over the whole land question arising from the great need our people have for land and our commitment to satisfy that need when in government."

Since the breakoff of the talks, the United States signalled its willingness to participate in an assistance effort. No money figures have been cited.

Britain and the United States are reluctant to give details on what has been offered because of failures of similar past efforts to bail out the whites. British sources said, however, that there had been no contacts with the Front on the issue since last Thursday and "there was nothing to add" to Carrington's statement then on the subject.

The three parties are to meet Friday morning to begin joint discussion of the transition to independence starting with elections.

Analysts are unanimous that the transitional issues will be much harder to settle than the constitution, which took 38 days to resolve.