Robert Mugabe, an avowed Marxist and leader of African guerrillas who fought a seven-year war to end white rule here, swept to a landslide victory today in elections for a black-majority government.

Mugabe's party, the Zimbabwe African National Union, won 57 of the 80 black seats in the lower house of Parliament, while his guerrilla ally, Joshua Nkomo, got 20 seats. Bishop Abel Muzorewa, the former prime minister, gained three seats in a startling reversal of his victory last year.

Mugabe received 63 percent of the vote and swept three of the eight provinces. Nkomo got 24 percent, mainly in his tribal base in the western part of the country, and Muzorewa was held to 8 percent, winning seats in only two provinces.

Analysts had expected Mugabe to win the most seat's but to fall considerably short of a majority, thus opening the way for an anti-Mugabe alliance including the 20 white seats already controlled by former prime minister Ian Smith's Rhodesian Front Party.

The overwhelming size of Mugabe's victory may forestall the possibility of a coup led by elements of the white-officered military. The only reason to attempt a coup, one of Mugabe's workers said, "is if you can get away with it" -- unlikely in light of the guerrilla leader's vote.

Apparently seeking to prevent such a move, Mugabe met with Smith, Gen. Peter Walls, head of the military, and police commissioner Peter Allums. No details were available on the talks.

Eddison Zvobgo, Mugabe's election director, called the victory "the end of a life's pursuit," but also warned, "If the whites are tired of war, they'd better accept the result."

Just as stunning as the size of Mugabe's victory was the overwhelming defeat of Bishop Muzorewa, who has been backed by the 200,000 whites in the estimated overall population of 7.3 million. In voting just 10 months ago, Muzorewa won 51 of the 80 seats. Mugabe and Mkomo boycotted that election which failed to gain international acceptance.

Last week's election is bound to gain worldwide recognition since most of the more than 200 observers monitoring the process have proclaimed it free and fair -- considering that it was held just two months after a cease-fire brought about a tenuous peace.

The British governor, Lord Soames, Gen. Walls and Mugabe all went on nationwide radio and television to appeal for calm. Mugabe's appearance, 13 hours before announcement of the results, was a tipoff of his victory since none of the other political leaders was invited to make an address.

Mugabe, whose forces carried the brunt of the war burden, appealed for "victors and vanquished" to "refrain from committing a single crime of violence in the course of our jubilation or as a result of our disappointment." v

"If you break the law," he warmed, "you will be arrested and punished for it."

Walls' appearance obviously was intended to reassure the many whites who feared a Mugabe victory most of all. He said the greatest enemy the people faced was emotion. He emphasized that he was speaking for "all the lawful forces of the governor," including the security forces and the guerrillas who have assembled in camps under the terms of the cease-fire agreement.

"Anybody who gets out of line or for whatever reason starts disobeying the law will be dealt with effectively and swiftly -- and I may say with quite a bit of enthusiasm."

Walls has met in recent days with Mugabe and his backers in Mozambique and has pledged not to mount a coup in the event of a Mugabe victory. There has been widespread sentiment in favor of a coup among some whites who are concerned about the possibility of a Marxist dictatorship under Mugabe.

The diminutive, soft-spoken Mugabe, 51, has pledged not to institute sweeping nationalization of private property but instead to have a phased transition to socialism to prevent disruption of the economy, which is closely tied to that of white-dominated South Africa.

Soames said, "There must be no violent action or reaction of any kind" to the election. He added that he would work to establish "a broadly based government capable of achieving reconciliation and overcoming the divisions of the past."

It was not clear whether he meant that the winner should seek to form a coalition, but the size of Mugabe's victory has taken the matter away from Soames since he is bound to call on the guerrilla leader to form a government. s

The clear-cut victory is likely to speed up the formation of the new government since it will not be necessary to form a coalition. Mugabe, in any case, is expected to invite Nkomo to join in forming a government.

The soundly defeated Muzorewa has frequently criticized the conduct of the election by the temporary British, colonial administration, saying that widespread intimidation and electoral malpractices by Mugabe's supporters made it impossible to hold a free and fair poll.

Muzorewa in recent weeks has refused to say he would accept the result of the election, despite a pledge by all leaders to do so in the peace agreement signed last December in London.

Government officials fear that the announcement of the result could lead to widespread demonstrations with the potential for violence.

Some parts of Salisbury were like an armed camp with armored vehicles patrolling the streets and military planes flying low over the city. Three armored vehicles were parked at one key intersection leading out of town, and dozens of troops, all white, stood guard, their rifles and submachine guns at the ready.

An early indication of the size of the Mugabe victory came in the providence of Mashonaland Central, where one observer of the counting said he only noticed one non-Mugabe vote in the first 100 ballots he saw.

Nkomo's strength mainly came in his Ndebele tribal area in the western part of the country.

Voting was on a proportional basis, with parties required to get at least 10 percent of the vote in a province to gain a seat.

The British election commissioner, John Boynton, gave his qualified endorsement to the polling as a generally accurate reflection of the wishes of the Rhodesian people.

Boynton said in certain areas "the high level of intimidation and presures on voters" probably "distorted the pattern of voting." He estimated that about a quarter of a million people lived in these affected areas -- mainly in the southeastern and south central parts of the country.

Despite this situation, Boynton said he felt that "in the country as a whole the degree of intimidation and pressure was not so great as to invalidate the overall results of the poll."

The House of Assembly, now elected, will play a key role in the upcoming choice of the far weaker upper house, or Senate. The 80 black members will choose 14 senators and the 20 whites will pick 10. A council of chiefs will choose 10 and an eventual president -- to be elected by the Parliament as a whole -- will choose six senators.

British officials announced that a phased reduction has begun in the Commonwealth forces monitoring the camps where the nationalist guerrillas are bivouacked. Only a small liaison force of 64 British military personnel will remain at the camps still open. Some have been closed after guerrillas were transferred to basic training camps to join small units of Rhodesian government forces in a preliminary effort to forge one national army.