Rhodesia Prime Minister Ian Smith, calling the latest Anglo-American proposals on the future of the rebel colony unacceptable and saying he needed a new mandate, tonight announced a general election for Aug. 31.

Smith dissolved the white-dominated Parliament and appealed for a mandate to seek an internal settlement on black majority rule that would give whites safeguards for their future.

The mandate would allow him to write a new constitution and set up a broad-based government containing black Rhodesians who were willing to assist in obtaining a peaceful settlement, Smith said in a television broadcast.

For white confidence in the future to be maintained, he said, Rhodesia could not have a one-man, one-vote system.

Smith's rejection of the Anglo-American peace plan combined with the expected mandate to carry out his own settlement in the August election would end more than a year of diplomatic efforts which began with then-Secretary of State Henry Kissinger's visit to Africa in April of last year.

Smith's move is seen as an attempt to outflank both right-wing whites and blacks fighting for majority rule.

The expected victory for Smith's Rhodesia Front party would help to silence rightist critics who have complained about his limited moves toward majority rule. It would also reinforce his efforts to negotiate a settlement [LINE ILLEGIBLE] country, freezing out nationalist groups outside the country fighting for an independent Zimbabwe, the African name for Rhodesia.

The election would presumably beheld under current franchise rules, meaning that a third of the 270,000 whites would be allowed to vote, but only 10,000 of the country's 6 million blacks, less than 2 hundreths of 1 per cent.

Josiah Chinamano, vice president of the opposition Africa National Council, dismissed the election as irrelevant. He said an election embracing the largely white electorate would offer no solution to the country's problems.

Chinamano accused Smith of deliberately sabotaging the Anglo-American effort and declared: "Those of us who have always said Mr. Smith has no intention of settling have proved to be right again."

In London, Britain's Foreign Office declined immediately comment about Smith's announcement pending a study of the full text.

In his broadcast, Smith said any settlement should provide for a limited franchise and special white representation in Parliament were necessary as well as the maintenance of the independence of the judiciary, the security forces and the civil service.

Smith said the recent visit to Salisbury of an Anglo-American negotiation team had indicated a "dramatic turn for the worse" in the effort to reach a settlement.

Smith said the team told him there was no hope of anything other than one-man, one-vote, and there would be no special political representation for whites.

Smith said he was also told that guerrilla troops would be given special consideration for inclusion in the government's security forces. Smith said he called the election because British proposals for a constitutional settlement were "unacceptable."

The Rhodesian leader said he was seeking a new mandate for an internal political settlement. Its main objectives were a new constitution and a broad-based government containing black Rhodesians who were willing to assist in achieving peace.

Another objective would be the removal of "any racial discrimination which is considered to be unnecessary or undesirable," Smith said.

Smith said he foresaw a new constitution for Rhodesia - if not agreed with Britain, then agreed internally - before the end of this year.

He was believed to be pinning his hopes on a settlement with Rhodesia-based nationalist leaders Bishop Abel Muzorena and the Rev. Noabaningi Sithole, who are strongly opposed to the Patriotic Front of Joshua Kkomo and Robert Mugabe. The front directs the guerrilla war from bases in Zambia and Mozambique.

"As part of this plan I envisaged the creation of a broad-based government incorporating those black Rhodesians who are prepared to work peacefully and constitutionally with the present government in order to establish a base from which we would be able to draw up our future constitution," Smith said.

Meanwhile, he declared, the government would step up its war effort against the guerillas. He said it was "prepared to adopt stronger and, if need be, ruthless measures."

Meanwhile Rhodesia's chief justice dismissed an appeal against a death sentence passed on a captured black nationalist guerrilla, saying it is neccessary to publicize the fact that Rhodesia hangs "terrorists."

Justice Hector Macdonald said that to reduce the sentence passed on Isaac Mabika, 28, to life imprisonment as suggested by his lawyer would "only give credence to the impression current among sections of the population that the authorities do not carry out death sentences."