Prime Minister Ian Smith announced tonight that he will offer counter-proposals to the Anglo-American mission during negotiations here Thursday over the Western plan to bring about black-majority rule in Rhodesia.

He added, however: "I would be misleading you if I were to express any undue optimism about this visit," adding further substance to reports that he intends to reject the peace plan.

Although the defiant prime minister did not specify what his proposals would contain, they are likely to center around the question of security forces under a new government - an issue that has raised controversy everywhere during the diplomatic travels through southern Africa of British Foreign Secretary David Owen and U.S. ambassador to the U.N. Andrew Young.

The Anglo-American plan reportedly calls for disbanding the white-dominated Rhodesian military force and disarming the black nationalist guerrilla movements in Zambia and Mozambique. The new government would form a new army.

The five presidents of black "frontline" nations and guerrilla leaders of the Patriotic Front fighting Rhodesia's white-minority government made their dissatisfaction with the plan clear this weekend, declaring that guerrilla forces should dominate a new army and oversee the transition of power. South African Prime Minister John Vorster, who has great influence over Rhodesia's white rulers, reportedly told the Anglo-American team yesterday that he could not go along with dismantling the Rhodesian army.

The Rhodesian government is clearly opposed to this part of the Western-backed proposal. Over the past week, several Cabinet ministers have warned that this aspect alone would force the government to reject the plan on the ground that the security of whites and general order could not be guaranteed.

Smith said generally that his proposals would focus on "principles fundamental to any settlement:

"That the retention of white confidence, white skills and white expertise was of paramount importance.

"That the retention of the existing security forces was essential for the maintenance of law and order.

"That Rhodesia must remain firmly in the Western camp."

In his final speech before Rhodesia voters go the polls Wednesday for what is probably the last election run by the white-minority government, Smith charged:

We find the present British and American administrations united in their apparent determination to encompass our downfall, irrespective of the consequences.

"To this end, they not only give tacit encouragement to the Russian-trained and Russian-equipped terrorists, but they do their utmost to prevent our obtaining the weapons which we need to protect our population against there murderers.

"Their lack of concern for the effects of terrorism on our black people seems to show that the British and American governments have no real and genuine interest in the welfare of Rhodesians, whether they be blacks or white."

In an angry mood, Smith added, "Their overriding objective is to avoid a confrontation with Russia in or over southern Africa and they will go to any lengths to do so."

Smith has indicated throughout the campaign that he is dissatisfied with the Anglo-American terms, pushing instead his own plan - an internal settlement with two moderate African leaders, Bishop Abel Muzorewa and the Rev. Ndabaningi sithole, in a vaguely outlined "broad-based government" of both blacks and whites.

[President Julius Nyerere of Tanzania, the leader of the "front-line" presidents, said in a television interview that he would reject any Anglo-American proposals that did not call for the replacement of the current Rhodesian army with one based on black guerrilla forces. The interview with the BBC was conducted before Nyerere's meeting Tuesday with Foreign Secretary Owen and Ambassador Young.]

Smith called for the Wednesday's election to test increasingly divided white political allegiance. He wants a mandate, amounting to at least 44 of the 50 white seats in Parliament, to prove that the 270,000 whites are ready for a settlement with Rhodesia's 6.3 million blacks rather than continued white domination, as the new ultra-right-win Rhodesian Action Party offers. Although he is expected to win, an opinion poll over the weekend showed 36 per cent of the voting population still undecided.

Rhodesians have largely accepted the probability that the Anglo-American plan will be rejected.

In fact, a growing number of whites have expressed the feeling that the plan is designed to be rejected, and that the British and American governments are committed more to establishing credibility in black Africa than to settling the problems of white-rule Africa.

Some are even expressing bewilderment as to why the Anglo-American team is bothering to visit Rhodesia when indications are growing stronger each day that neither southern Africa's white leaders nor its black nationalists are prepared to accept the plan.