Patriotic Front guerrillas agreed today to let Rhodesian whites hold 20 percent of the seats in Parliament, paving the way for an agreement with Zimbabwe-Rhodesia's Prime Minister Abel Muzorewa on a new constitution.

Whites compose 3.5 percent of Rhodesia's population of 7 million and how much political power they would retain in any negotiated settlement has been a major obstacle to agreement.

The Patriotic Front concession at the British-sponsored talks on Rhodesia's future removes the guerrillas' key objection to the British-proposed constitution that the Muzorewa government had accepted in principle Friday.

British spokesman Nicholas Fenn said that his government "welcomes" the Front's move and described it as "significant progress."

The two warring sides are still a long way from agreement, however, on an overall settlement to end the 14-year-old independence problem even if they agree on the constitution.

Still to be settled are difficult issues involving formation of a transitional government, disposition of the rival military forces and a cease-fire leading to an election.

For more than a week the Front had sharply criticized the British insistence on top-heavy white representation as racism. The explanation for the sudden turnabout apparently involved deft British use of pressure.

Neither faction can afford to be the wrecker in this conference since there is pressure from each side's supporters to try to reach a settlement to end the escalating seven-year-old war that has already caused more than 20,000 deaths.

It was the Front's turn to show flexibility after Muzorewa beat back protests from former prime minister Ian Smith, who has great influence over whites in Zimbabwe-Rhodesia, and agreed Friday to the British proposals, which sharply cut back the current ability of the 230,000 whites to control the government.

Looming in the next two weeks are rightist efforts in the U.S. Congress and at the British Conservative Party conference to try to lift economic sanctions against Muzorewa's government, which has never gained international recognition.

The Front could hardly afford to remain intransigent over the constitution with Muzorewa demanding to know what more the world wanted of him after agreeing to the reduction in white rights.

The two moves mean that for the first time in the conference, which entered its third week today both sides have shifted toward the middle-ground stance held by the British and away from long deadlocked constitutional positions.

A British source predicted that it would be possible to get agreement on the overall settlement by the time the Conservative Party conference starts Oct. 9 and warily estimated that at the present pace perhaps the halfway point of the conference had been reached. The British have set no deadlines but Parliament faces one Nov. 15, when it must decide whether to renew sanctions against Salisbury.

The negotiations "are definitely on the upswing," the source said. Lacing his remarks with a caution developed over countless past failures on the issue, he added: "We are beginning to establish the sort of momentum that can lead up to significant developments" toward a settlement.

However, he continued, "no one is under any illusions. The transitional arrangements will be the most difficult part of the conference."

What the Patriotic Front did today was to present a complex amendment, with two alternatives, to proposals for the legislature in its draft constitution. Both alternatives call for 96 black and 24 white seats, unlike its original draft which did not allocate seats on the basis of race.

In its preferred alternative all eligible voters would get to vote for the black seats and blacks would join with whites in voting for nine of the 24 white seats. In the other alternative there would be separate white and black rolls to elect members of the two races.

The second alternative would be preferable to the main white party, Smith's Rhodesian Front, since it would allow the party to maintain its dominant role among whites.

Patriotic Front spokesman Eddison Zvobgo said: "It is with sadness that we have to present a thing like this. It jars our every sense of justice but since the other parties wanted it, we have no alternative but to make these proposals under protest."

That statement alone represented the pressure the Front felt since in the past it often has declined to shift despite opposition from other parties -- just as Smith frequently did over the last 14 years of frustrating negotiations.

Zvobgo said the Front would seek to change racial requirements for seats if and when it gains power and after whites see their "imagined psychological fears are groundless."

In keeping with the conciliatory atmosphere, British spokesman Fenn said, "I acknowledge the realism which has led the delegation to this step."

He declined comment on the substance of the proposal until it could be presented to the next meeting with Patriotic Front leaders Robert Mugabe and Joshua Nkomo, probably on Wednesday.

Fenn did differ with the Front's interpretation of the representation as "racist," instead of calling it "realist, in the interest of national reconcilation."

Other remaining differences between Britain's and the Front's proposed constitutions include entrenchment for five to 10 years of the representation rights, the number of white seats in the basically rubber-stamp Senate and its powers to delay legislation and the powers of the presidency.

British sources said, however, that all these problems should be solvable now that the Front has conceded the key representation issue. The feeling was that having given in on the most important part, there would be little point in making a major issue out of less significant matters.

Deputy Foreign Minister Ian Glmour chaired today's meeting in the absence of Foreign Secretary Lord Carrington, who is in New York to address the United Nations General Assembly.