Rhodesian Prime Minister Ian Smith took what is probably his biggest political gamble since leading the rebellious break from Britain 12 years ago when he dissolved his government today and opened the way for a test of political allegiance among the increasingly divided white population.

Blacks and whites alike in this troubled southern African territory were confused and surprised by Smith's unexpected announcement last night, in which he also indicated that he is rejecting the latest Anglo-American settlement proposals to end Rhodesia's constitutional crisis and five-year-old guerrilla war because they demand too much.

The implications of last night's nationwide broadcast are ominous for both Rhodesia's international and domestic status:

By rejecting the Anglo-American deal - which called for immediate one-man, one vote elections - Rhodesia now stands no hope of gaining international backing for any kind of settlement deal with its dominant 6.3 million black population.

By calling for a general election Aug. 31, Smith has in effect acknowledged that his government is paralyzed by the recent breakaway of 12 members of Parliament who have formed a new right-wing movement - the Rhodesia Action Party - opposed to black rule.

Most whites believe that Smith's Rhodesia Front Party will win most of seats, although they doubt that he will get the numbers needed to enforce his own policy. The result would be to leave Rhodesia divided and paralyzed as never before.

Smith clearly made the two moves because of troubled white morale. He knows that most whites would not support the Anglo-American deal since it offers few guarantees for white political and economic security.

Smith also recognized that the long-anticipated opposition to majority rule finally had surfaced when the 12 members of Parliament bolted his party, crippling his government. The 58-year-old leader, who has ruled the country for more than 14 years, was forced to call the election to reaffirm his leadership.

In his speech, Smith appealed to Rhodesia's 270,000 whites for a "clear mandate" that will allow him to proceed with his own settlement plan, most likely with two moderate nationalist leaders. Bishop Abel Muzorewa and the Rev. Ndabaningi Sithole.

Smith's plans drew scorn from moderate and militant blacks and expressions of disappointment from diplomats seeking to work a compromise, news agencies reported.

In Salisbury, both Muzorewa and Sithole said they would accept nothing less than one-man, one-vote. Muzorewa has said in the past that he would not cooperate with Smith on a so-called "internal" solution and Sithole said Monday that any black who joins Smith would be a "puppet."

[Robert Mugabe, a leader of guerrilla forces fighting Smith, said in Dares Salaam, Tanzania, that the election call is in "irrelevant development in the struggle for freedom."]

[In London, British Foreign Secretary David Owen called Smith's moves a "great tragedy," while U.S. Ambassador to the U.N. Andrew Young labeled them "another desperate move" by the Rhodesian.]

Smith's plan calls for a gradual transfer of power with a limited franchise for Africans, continued white domination of the military and special white representation in Parliament.

A clear mandate would require Rhodesian Front party victories in 44 of the 50 white constituencies, the minimum required to change the constitution. The Front now has 37 seats.

The gamble is that the move could backfire on Smith. There is growing dissention among whites over several issues:

The implications of Smith's plans for an internal settlement deal, since it offers few guarantees for whites without international support. In effect, it will amount to new black faces in government with the same problems: a guerrilla war, a troubled economy boycotted by the outside world, and political isolation.

The apparent lack of concern about the status of civil servants, triggered first by a recent wage freeze and then by the implications of even a partial transfer to black rule. Many white civil servants would be replaced by Africans. This could have a great impact on the election, since there are about 28,000 white civil servants among the 85,406 registered white voters. Only about 10,000 blacks have the franchise out of 6.3 million.

Soaring inflation and taxation, prompted by the rapidly escalating defense budget needed to fight the guerrilla war. The sales tax was recently increased from 10 to 15 per cent, while inflation is a record 13 to 14 per cent.

The fixed prices of farm products, which has hurt another important section of the white constituency.

Underneath it all is the fear that, after 12 years, the country is no closer to ending its problems than it was in 1965.

The new right-wing party has described the prime minister's policy as vague, aimless and drifting - a strong appeal for many whites who have struggled through a dozen years of crises.