Britain opens a conference here Monday that is likely to decide with finality whether the embattled former white-ruled nation of Rhodesia can become an internationally accepted black-ruled Zimbabwe through negotiations.

The alternative is tens of thousands more deaths in Africa's bloodiest war for independence.

British Foreign Secretary Lord Carrington, flanked by rival claimants to power who will refuse to recognize each other's presence, will open the constitutional conference in a spirit of guarded optimism.

It is optimism engendered mainly, however, by the apparent exhaustion of most of the parties involved in the 15-year-old struggle that has been dominated since 1972 by spreading guerrilla war.

Just how difficult Carrington's task will be was illustrated today when Bishop Abel Muzorewa, Zimbabwe Rhodesia's prime minister, and his arch opponent, guerrilla leader Joshua Nkomo, arrived at London's Heathrow Airport almost simultaneously and heaped abuse on each other from a distance.

A British official noted that protocol aides went to considerable lengths to see that the two leaders' paths did not cross.

Nkomo, who seems to relish an adversary role with the British press, called Muzorewa a "bandit" and "an ignorant bishop" who had been unfairly taken advantage of by the whites. He added derisively, "If the bishop is present he will help us to conduct prayers."

Standing nearby taking notes was Muzorewa's press spokesman, who had just finished running the bishop's press conference.

Muzorewa avoided mentioning Nkomo or Robert Mugabe, co-leaders of the Patriotic Front guerrillas, by name. His target, however, was clear, as he said their forces were terrorizing citizens "by committing atrocities too horrible to mention, by murder and massacre . . . to impose powerhungry individuals with foreign Marxist ideologies."

Only about 15 feet of space between tables -- and the British delegation -- will separate the warring factions when the conference opens Monday in the ornate drawing room of the Victorian-era Lancaster House, scene of numerous independence negotiations for former British colonies.

Both sides will carry out the fiction that the other is not there and that they are simply dealing with Britain, still legally the colonial power with the ability to grant independence.

In effect, Carrington will be presiding over "proximity talks," like President Carter at Camp David, but with the difference that the parties are all in the same room.

Carrington's task will be to rein in the rhetoric and try to keep the Zimbabwe-Rhodesian government and the Patriotic Front focused on reaching agreement on a new constitution to lead to independence.

Most observers believe that this ninth effort in the past 13 years to bring about a negotiated settlement will be the last. Tanzanian president Julius Nyerere, leader of the front-line states supporting the guerrillas, said last month that if these talks do not succeed the only solution will be fighting the bloody seven-year-old war to its conclusion.

Escalation of the war, which already is killing upwards of 100 persons a day, might also lead to increased East-West competition in southern Africa, with the guerrillas turning more toward the Communist-bloc countries.

It would also pose a dilemma for U. S. relations with black Africa, since Congress is pressing the Carter administration to recognize the Muzorewa government and lift economic sanctions. So far black Africa has stood firm in opposing Muzorewa, who was elected in April under a controversial constitution that grants significant power to the 230,000 whites in the country's population of 7 million.

Nyerere, who played a key role in bringing about the new British initiative at last month's Commonwealth conference in Lusaka, Zambia, has decided evidently to play a direct role in monitoring progress of the London talks. He arrived in London tonight from the nonaligned conference in Havana enroute to a state visit to Ireland next week. He also is scheduled to return to London next weekend.

The Tanzanian president and his Zambian counterpart, Kenneth Kaunda, are known to be anxious to see a Rhodesian settlement because of economic problems. They have indicated they are ready to pressure their guerrilla clients.

Nkomo's guerrillas are based in Zambia and Mugabe's get aid from Tanzania, so the British are hoping to use this potential for pressure to effect a breakthrough in the negotiations.

Less certain is the role of Mozambique, the other key front-line state and Mugabe's key supporter. So far it has been silent on its attitude toward the British initiative, although it, too, has severe economic problems that would be eased by peace.

Just today the Zimbabwe-Rhodesian military announced that it had ended a three-day offensive against guerrilla and Mozambican forces deep inside Mozambique after inflicting heavy casualties and causing widespread destruction, including strikes at road and rail links.

Carrington's plan is to concentrate on the constitutional issues first, seeking agreement on a charter removing or easing elements of white control. He would then proceed to tackle the tougher issues of agreeing on a cease-fire, disposition of the rival military forces and election of a new government.

Past efforts often have failed because all the issues have been put up for grabs from the beginning, British officials say they believe. At this point they are refusing to discuss issues beyond the constitution.

Indicative of past differences is the fact that the country is still plagued by controversy over its very name. It may become the first country ever to have three names in one year -- going from Rhodesia to Zimbabwe-Rhodesia to simply Zimbabwe.

Whites named the country after British colonialist Cecil Rhodes and this was maintained during nine decades of white domination.

Muzorewa, after initially agreeing to the double name, is proposing to change it simply to Zimbabwe, the African name for the country, in a move to try to win black support.