Zimbabwe-Rhodesian Prime Minister Abel Muzorewa announced today that his government has accepted in principle British proposals for a new majority-rule constitution, and as a result he demanded the immediate lifting of international economic sanctions.

The move was the first breakthrough in the 12-day-old settlement conference sponsored by Britain, but there are many more hurdles blocking a settlement of the intractable problem that began when Rhodesia illegally declared independence in 1965.

The rival Patriotic Front guerrillas, who will now come under increased pressure to accept the British proposals, discounted the agreement. A spokesman for the group fighting an escalating bush war in the southern African nation said peace and an end to sanctions can occur only when his organization agrees to terms.

Britain also made it clear that sanctions would not be lifted now as a result of the Muzorewa government's acceptance of "the general principles" of the British constitutional proposals.

"There is no way the demand they make can be satisfied immediately because if it was it would torpedo the conference," a British source close to the conference said. He referred to acceptance at the beginning of the conference of agreement linking a constitution for a new government to accord on transitional arrangements to implement it.

Muzorewa announced that the delegation agreed to accept the British proposals in a secret ballot in which the vote was 11 to 1. He refused to answer questions, but there was little doubt that the lone dissenter was white former prime minister Ian Smith, who has held out all week for retention of a white blocking power over constitutional changes.

The British proposals provide for removal of the blocking mechanism, reduction in the percentage of white representation in the Zimbabwe-Rhodesian Parliament and removal of white authority over commissions controlling the military, civil service and judiciary.

Smith has been the hero of the white minority community of 230,000 since declaring independence 14 years ago, and there was concern that his reported refusal to go along with the rest of the delegation could lead to an undermining of white confidence in the Muzorewa government.

Smith and Muzorewa negotiated the current Zimbabwe-Rhodesian constitution which retains considerable control over the government for whites, who already are fleeing the war-torn country at the rate of 1,000 a month. The government depends heavily on the whites, who make up only 4 percent of the 7 million population, since they form the backbone of the military fighting the war and control the economy.

British Foreign Secretary Lord Carrington welcomed the agreement, calling it "a major advance."

"I pay tribute to the constructive attitude which has enabled Bishop Muzorewa to take this step," he said. "We now hope for agreement also with the delegation" of Patriotic Front leaders Robert Mugabe and Joshua Nkomo.

Muzorewa said his government's "acceptance is subject to the satisfactory working out of the many details concerned with these proposals and the subsequent steps that will then have to be taken."

He offered no elaboration but the Patriotic Front and British sources took his vague language to mean that he has accepted the need to agree on arrangements involving a transitional government, disposition of the rival military forces, a cease-fire and elections to implement the constitution.

He called the acceptance "a major contribution toward finding a reasonable, just and international acceptable solution to our current problems" and added "we now demand, and it is our right to do so, that the sanctions against us be lifted immediately since they no longer hold any validity."

His controversial government, he said, was black and elected by a majority of the population and thus "there is no longer moral or legal grounds for the continuation of sanctions" first imposed against the Smith government in 1966 by the United Nations.

Patriotic Front spokesman Eddison Zvogbo said Muzorewa "was living in Alice in Wonderland" if he thought this agreement would bring peace. Zvogbo said this first step would "remain a dead letter unless implementing machinery is also agreed to."

Continuing the sharp personal attacks on the bishop by the Patriotic Front, the spokesman said Muzorewa was "like a little kid asking for sweets" in demanding the end of sanctions.

"He knows he is a representative of a minority regime. He knows he is a puppet."

The agreement was reached in bilateral talks between the British and the Muzorewa delegation. Similar separate talks are continuing with the Patriotic Front.

Zvogbo said he could not say how close the Nkomo-Mugabe forces were to agreement with the British. He added that the Front was being patient and "time was not of the essence."

British spokesman Nicholas Fenn merely said the talks were "in train" and would continue Monday. Yesterday he admitted that "wide differences" separated the two sides.

The main problem is in the area of parliamentary representation where the British want 20 percent of seats reserved for whites. The Patriotic Front has said there should be no seats based on race.

There are strong indications, however, that the Front is willing to give in on its constitutional demands since the transitional arrangements are more important to it.