Rhodesia's embattled multiracial government yesterday offered an unconditional amnesty to black nationalist guerrillas fighting against its security forces, and appealed to them to lay down their arms and return home because "majority rule . . . has now been achieved."

The Executive Council, composed of Prime Minister Ian Smith and three moderate black leaders, also lifted the ban in force since the early 1960s on the two political parties headed by guerrilla leaders Joshua Nkomo and Robert Mugabe, and invited them to join the transition to black majority rule.

"We stress once again that nobody is barred or excluded from the process," the government declared. "Those outside the country are free to return and play their part under the amnesty, provided only that they come in peace."

In embarking on the most crucial phase of their experiment, Smith and the three black leaders who joined him in the interim government - Bishop Abel Muzorewa, Chief Jeremiah Chirau and the Rev. Ndabaningi Sithole - seemed to be trying both through language and deed to make their amnesty as appealing as possible to the guerrillas. The guerrillas have firmly rejected all such efforts in the past by the previous white-minority Smith government.

The new government also appeared to be trying to satisfy some of the conditions that the guerrillas had set as prerequisites to acceptance of any safe-return plan, including the release of political detainees.

If their appeal is ignored by the bulk of an estimated 7,000 to 8,000 guerrillas now operating inside Rhodesia and along the country's borders, the chances of the "internal settlement" succeeding would be sharply diminished.

However, if the government's offer of a condition-free amnesty to the guerrillas is accepted by large numbers and the six-year-old war begins to wind down4, it will give confidence to both blacks and whites and enhance the legitimacy of the multiracial government that is presiding over the transition to full majority rule.

An effective cease-fire would also make it possible for the current government to proceed with plans to hold elections for the new government that would take power Dec. 31 when Rhodesia would become the new black-ruled nation of Zimbabwe.

Opinion at this point is sharply divided over whether the crucial safe-return and cease-fire effort will succeed. Both Muzorewa and Sithole have predicted that they will be able to convice the guerrillas to return and support the transitional government.

Many observers, however, believe Muzorewa and Sithole are greatly overrating their ability to win the confidence and backing of the guerrillas. "Anyone can dream," one black journalist remarked.

Well-informed sources said that during initial discussions between the guerrilla leaders and the Muzorewa and Sithole groups, the guerrillas had laid down four demands.

One was that the political ban be lifted on Nkomo's Zimbabwe African People's Union (ZAPU) and Mugabes Zimbabew African National Union (ZANU) - a step that was taken yesterday.

A second condition demanded by the guerrillas was that the Rhodesian government release all political detainees, and in yesterday's statement, the Executive Council said it has released 700 political detainees since taking power. It said the cases of the remaining 200 "are being examined."

"The only provision we make is that after their release, they should live peacefully and not engage in unlawful activities," the statement said.

A third guerrilla demand was that the government dismantle the 200 protected villages where it has forcibly resettled about 500,000 rural peasants so that guerrillas could not live with them or be fed by them.

The government document yesterday admitted - for the first time since Smith's old white-supremacy government established the villages - that if realized "these protected villages are unpopular, and that the people prefer to live freely without restrictions in their own areas.

"As the fighting dies down and peace is restored, the protected villages will be dismantled," the government promised.

As for the fourth demand, yesterday's statement made no reference to an end to hangings for politically motivated crimes, possibly in hopes of not inflaming white public opinion, which has for the most part regarded the guerrillas as Communist-inspired murderers. But government officials have said previously that these have stopped.

There were also a number of telling details in the language of yesterday's statement, however, which illustrated the narrowness of the tightrope the interim government walks.

The document, for example, delicately avoided using the words "guerrilla" or "freedom fighter" - the terms blacks use for the nationalist fighters - or the standard white formulation, "terrorist." Inside, it referred to the men in the bush as "our followers."

In a clear effort to persuade the guerrillas that the four men serving on the Executive Council are equals, the government statement did not refer to Ian Smith as Prime Minister Smith - even though he still holds that title - but simply as "Mr. Smith."

And in urging the reduction of civilian casualties, the document implicitly acknowledges for the first time publicly that some of these have been caused by actions of the Rhodesian security forces. It states that "those on both sides who are doing the fighting should now make certain that civilian targets are avoided."

Yesterday's statement, however, made no mention of a plan favored by Sithole for offering material incentives to the guerrillas - sort of a G.I. Bill - to lay down their arms.

Supporters of Sithole said, however, that a plan that would provide each guerrilla who returns with assistance to complete his education and to obtain jobs and loans for home and farm purchases is still under discussion by the interim government.

Yesterday's appeal follow contacts over the past month between officials of both Muzorewa's and Sithole's parties and guerrilla leaders mostly in the eastern part of the country.

During that period, Rhodesian security forces, according to government officials, were told avoid contacts with guerrillas in some areas as an indication to them that a new situation has arisen.

"We know from our contacts with them that most of those who have been fighting for the principle of majority rule are aware that the battle has been won . . . We guarantee their safety if they come in peace. Through our contacts with them, detailed arrangements are being made for their reception and to enable them to return to their homes and rejoin their families if they wish to do so," the statement said.

No details were released on the implementation of the safe-return call but Sithole said it would be communicated to the guerrillas by radio, pamphlets and in person.