Prime Minister John Vorster announced at a hastily arranged press conference today that the Parliament will be dissolved immediately and nationwide elections held on Nov. 30.

The move is clearly designed to refute mounting criticism, both internal and international, by proving that his beleaguered government holds the support of the majority of South African voters.

Vorster began the surprise announcement that elections would be held 18 months early with a blast at outside medding:

"What I am asking the electorate to do is to say whether they agree with my standpoint and that of my ministers that no other country has the right to interfere in our affairs or to prescribe to us."

The surprise move followed to Cabinet meeting at which the growing list of problems facing the country were debated: the death in detention of black activist leader Steve Biko, the possibility of further U.N. sanctions, continued nationwide unrest, the dispute over Namibia (Southwest Africa) and South Africa's role in the Rhodesian crisis.

Although the controversial death of Biko and the role of Justice Minister Jimmy Kruger in the case were apparently involved in the Cabinet discussions, there was no information available to the substance of the talks.

In response to a question, Vorster said that the calling of the election "has nothing whatsoever to do" with Biko's death.

Vorster is under pressure from many quarters, however, including a significant number of the country's dominant Afrikaners, to remove Kruger from the Justice Ministry. There was speculation in some quarters that forming a new government would provide a convenient way of at least shifting him to another ministry.

The recently troubled past is not the only issue in the election. Vorster said today that he will test the white electorate - which numbers about 2.2 million - on the government's plan for the future: restructuring the government through a new constitution that would provide separate parliaments for South Africa's 2.3 million "Coloreds" (persons of mixed race) and 750,000 Asians. The 18 million blacks are to be resettled in "homelands."

Neither the "Coloreds" nor the Asians have any vote. The blacks are officially viewed as citizens of their homelands" and do not vote.

Dr. Nthato Motlana, a leader in Soweto - the black gehtto outside Johannesburg, and one of the country's most troubled areas - denounced the election, saying: "While politics and white elections don't mean a damn thing to the black man."

While Vorster claimed today that the new constitution would give "meaningful rights and privileges to colored peoples," there would in the end be no serious deviation from the current policy of apartheid, or separate development of the races.

Many observers and political analysts here beelieve that South Africa is facing its most critical period since the ruling National Party took power in 1948. Since the first racial unrest 15 months ago, there has been a noticeable growth in dissidence even among formerly staunch government supporters.

The mood of uncertainty here is unparalleled, with emigration figures increasing as whites wonder about their security, either under the current white government - which so far has not been able to end the internal crisis - or a future, possibly embittered, black government.

The National Party, which won the last general election in April 1974 with it biggest majority since rising to power, "no longer has the same mandate," one editorial writer commented tonight. "Vorster had to do it to prove the tide isn't turning against him."

An editorial in last week's "To the Point" magazine, a South African weekly, pointed out: The National Party's 1974 mandate - while it still has almost two years to run - is under some challenge in the crucial areas of race and group relations, and in foreign policy. That challenge is not so obvious in the parliamentary division of parties as it is in the debates both inside and out of party circles, in academia, commerce and the lower levels of political discussion throughout the country."

At stake at the November election are 171 parliamentary seats. Vorster's party now dominates heavily, with 123 seats. The largest opposition group is the new Republic Party - formerly the United Party - with 24 seats. The most moderate white political group is the Progressive Federal Party - until recently the Progressive Reform Party - with 18 seats. The South African Party, a small conservative fringe movement, has six seats.

Despite the country's growing problems, the Afrikaner-dominated National Party will probably manage to maintain its lead, while the fragmented opposition may finally sort itself out with one party emerging with a strong voice to argue on behalf of moderate whites.

Vorster struck a defiant tone at the packed press conference today, reminding listeners that he and other ministers in his government have spoken out strongly against demands for "one-man, one-vote".

"We feel, however, that the time has come and that it is only too right for the electorate too to add their voices to this protest," Vorster said, adding that this is especially important because he and others in the Cabinet have been pictured in "certain propaganda" as softening their positions on opposition to enfranchising South Africa's nonwhite population.

This is the third time Vorster has called a snap election. In 1970 he called for a vote to block an extreme-right splinter group from taking over the National Party. In 1974 he called an election when a military takeover in Portugal led to decolonization moves in Mozambique and Angola. In that vote, he sought an mandate for his "detente" policies with black Africa.