Both the government of Zimbabwe-Rhodesia and guerrillas who oppose it reacted cautiously but with hostility to day to the proposal for the country worked out over the weekend by Commonwealth leaders meeting here.

The formula for the southern Africa trouble spot also drew criticism from the right wing of British Prime Minister Margaret Thatcher's Conservative Party.

Thatcher, however, pledged to "move quickly" to try to achieve a cease-fire in the embattled country and called the Coqmonwealth plan approved last night a "very much better prospect for peace."

Defending the surprise agreement, Thatcher warned that such a chance for a peaceful solution to the 15-year-old Rhodesian problem "might not occur again."

In Salisbury, Prime Minister Abel Muzorewa said Thatcher's agreement to work for a new constitution and new elections was "totally unfair and in fact an insult." If such moves were implemented, they would bring an end to Muzorewa's government, which came to power in controversial April elections carried out by the former white-minority administration.

A spokesman for one of the guerrilla factions fighting Muzorewa's government, the Zimbabwe African People's Union of Joshua Nkomo, was equally critical in an initial statement.

"The assumption that the present British government is impartial and is therefore the fitting authority to conduct the elections in the country is not acceptable to us," the statement said.

In London, Julian Amery, leader of a right-wing bloc of Conservative parliamentarians who strongly support Muzorewa's government, minced no words about his opposition to the policies of his party's leader. She has jeopardized "the most hopeful experiment in a multiracial democracy that the African continent has ever seen," he said.

Thatcher said at a press conference "we do not anticipate any British troops going to Rhodesia" to help supervise elections because "by that time we should have gotten a cease-fire."

She admitted, however, that it was highly doubtful that a truce in the six-year-old guerrilla war could be achieved before a constitutional conference, which will probably be scheduled next month in London.

"One has to show some progress toward" a peaceful settlement first, she added.

How to maintain a cease-fire without outside troops in a country where tens of thousands are bearing arms was just one of many unanswered questions left by the Commonwealth agreements.

Other problems involved disposition of the Rhodesian military and guerrilla forces and the transition to an elected government.

Thatcher refused to be drawn out on the subject, telling reporters "you're trying to get far further than we can at the moment."

Cryptically referring to the difficulties of reaching total agreement, she said: "if you knew it all before you had it, you wouldn't need to have it."

Tanzanian President Julius Nyerere, in a separate press conference, voiced cautious optimism over the agreement reached by Britain and its former colonies, but he warned that there would not be another such chance to achieve peace.

"This is absolutely the last chance to gain a negotiated settlement" at the biennial Commonwealth conference, said the chairman of the Front Line states supporting the guerrillas.

"By 1981, Rhodesia either will be independent through this agreement or there will be no possibility of talking," Nyerere said. War, he added, will be the only avenue for independence if it has not been achieved peacefully by then.

He said the Patriotic Front guerrillas are not the problem in trying to reach a settlement but rather the white minorities under the leadership of former prime minister Ian Smith. "What happends if Smith doesn't accept?" he asked. "Then we en we are back to square one."

Nyere met today with representatives of the guerrilla groups and said he presented the commonwealth plan to them as a "reasonable" proposal. A Patriotic Front spokesman said the group would hold a press conference Tuesday.

Nyerere, however, cautioned against instant reaction. "One needs to give the Patriotic Front time to sit down and give a considered reaction." He noted that the two leaders of the Front, Robert Mugabe and Nkomo are abroad.

Like Thatcher, Nyerere refused to be drawn out on details of how Britain might proceed.

"We have got Britain to accept certain undertakings. We say fine, now go and carry them out," he said. Meanwhile, he said, "we will continue to support the Patriotic Front to go on fighting."

"Clearly," he added, "there are not going to be changes about recognition or lifting of sanctions until an acceptable settlement has been achieved."

Nyerere declined to speculate whether Thatcher had changed her policy of tacit support for Muzorewa as has been charged by some Conservative elements in Britain. Other Commonwealth diplomats adopted a similar stance, perhaps seeking to help Thatcher overcome the expected domestic political criticism.

An Australian source close to Prime Minister Malcolm Fraser, who played a key role in the weekend negotiations leading to the agreement, said the Commonwealth helped to provide an umbrella to allow Thatcher to move further than she would have been able to do alone domestically.

Thatcher and her aides continue to maintain that British agreement to the Commonwealth plan did not represent change in policy but merely an evolutionary process. She said the African nations which had criticized her earlier had misunderstood her intentions.

"I was never as hard as they thought," she said. CAPTION: Picture, MARGARET THATCHER...will "move quickly"