Leaders of the five "front-line" African states backing Rhodesian nationalist guerrillas gathered here today for a crucial summit meeting to hear the details of the latest Anglo-American plan for a peaceful settlement of the Rhodesian dispute.

The plan is to be formally presented Saturday morning by British Foreign Secretary David Owen and U.S. Ambassador to the United Nations Andrew Young, who arrived together tonight on a special British air force jet from the Nigerian capital, Lagos.

Speaking to reporters at the airport, the British diplomat said he had come to present a set of "serious and well-thought-out proposals" that, when published late next week, would be seen to be "fair and reasonable."

Owen said he does not expect a "yes-or-no answer" from the "front-line" leaders this weekend, and warned the press to suspend judgement until the proposal are made public.

Owen and Young refused to accept their mission as one more "hopeless" attempt to resolve the intractable 17-year-old Rhodesian dispute, which began in 1965 when the 270,000 whites there unilaterally declared their independence from Britain.

"There is no final attempt," remarked Young. "It's only failure when you quit," he added, comparing the diplomatic offensive to protracted warfare.

The Anglo-American proposals are transition administration under a British administrator general prior to the establishment to a government repre-understood to provide for a neutral senting the territory's 5 million blacks through election in Rhodesia.

The white prime minister, Ian Smith would step down before hand. Also, the white-dominated Rhodesian army would be disganded and some black guerrilla units disarmed, while with an international peacekeeping force would play a key role in the transition period.

Also arriving today were the two leaders of the black Rhodesian Patriotic Front, Joshua Nkomo and Robert Mugabe. They will be the first to be formally briefed by Owen and Young Saturday morning.

Nkomo and Mugabe, who have already been given the broad outlines of the proposals, were extremely cautious in their public remarks.

"You know how many time initiatives have gone under before," Nkomo told reporters at State Lodge, a rest house 10 miles outside the capital where the two black Rhodesians met with the five "front-line" leaders this afternoon.

"It's a British initiative and the tradition is that they are all ineffective," remarked Mugabe.

Nonetheless, neither leader was ready to dismiss the latest and most carefully prepared Anglo-American diplomatic effort out of hand, despite their caustic remarks and skepticis.

[In Wankie, Rhodesia, Smith rejected any replacement of Rhodesian armed forces by international peace-keeping troops, UPI reported "I can't believe that any person in his right senses would advocate that," Smith said.]

Attending the summit meeting are President Julius Nyerere of Tanzania, Kenneth Kaunda of Zambia and Samora Machel of Mozambique. Representing Angola is Jose Eduardo dos Santos, the first vice Premier, and Botswana sent Vice President Quett Masire.

Angolan President Agostino Neto is on a state visit in Cuba and Bostwana's President Seretske and Khama is reportedly ill with a heart condition in a London Hospital.

The five "front-line" states are all deeply involved in the Rhodesian dispute either because they border on that white-ruled country and the war has spilled over into their territories or because they provide training and arms for the black nationalist guerillas.

All five countries have endorsed the Patriotic Front against two other Rhodesian nationalist factions headed by Ndabainingi Sithole and Bishop Abel Muzorewa.

Neither Sithole and Muzorewa was invited to attend the meeting here, and their views on the British-American plan are not known for certain. Nonetheless, any plan that provides for free and unrestricted elections for a black-majority government is likely to gain their support, since neither has a guerrilla army.

One touchy issue in the Anglo-American plan is understood to be a provision for disarming part of the Patriotic Front's guerrilla army. It is unlikely that either Mugabe or Nkomo will agree to this and uncertain whether the "front-line" leaders would be willing to force them to accept.

Washington Post Corespondent Jonanthan C. Randal reported the following from Lagos:

Young and Owen left here on the first leg of what Young predicted would be an "exhaustive and exhausting" effort.

Buoyed by a productive, two-hour meeting with Lt. Gen. Olusegun Obasanjo, the Nigerian head of state, they flew to Lusaka.

Owen briefed Obasanoj in detail on the proposals he will also take later to South Africa and Rhodesia - a mark of defence due this oil-rich country, which is the major source of funds for nationalist guerrillas fighting the white-minority government in Rhodesia.

Reports that a Nigerian military mission would leave Saturday to examine the rival Rhodesian guerrilla organizations with an eye to forging them eventually into a future army for an independent black majority government underlined Nigeria's growing role.

Young and Owen indicated that they are not permitted by the negative reactions already expressed by Rhodesian Prime Minister Smith and those they could expect from the black Africans involved.

Earlier, more than a hundred countries at a U.N. meeting here adopted by acclamation a tough, 34-point declaration on pledging an arms embargo against South Africa and an end to civilian and military nuclear cooperation in southern Africa.

The United States, Britain, Canada, France, West Germany and some other Western European countries expressed reservations about these and other aspects of the declaration published at the end of the week-long U.N. sponsored anti-apartheid conference.

Spokesmen for South African liberation movements expressed "complete satisfaction" with the document .The results represent an unquestioned victory for radical Africans and their al-Mes among among the other Third World and Communist countries.

Denmark, Ireland and the Netherlands sided openly with the radicals, to the apparent discomfort of their European Common Market partners.