Under the protective guns of 18,000 South African troops, blacks and whites in Namibia (Southwest Africa) began going to the pools today in the territory's first multiracial elections.

Voting was steady and no violent incidents occurred, according to South African officials, on the first day of this five-day exercise, which has been condemned by the United Nations and threatened by the black nationalist guerrilla movement, the Southwest Africa People's Organization (SWAPO).

In addition to the troops, South African Air Force Mirage jets were on standby in the northern part of the country where SWAPO guerrilla activity is most intense.

Electoral officials characterized the voting among the population of 100,000 whites and 900,000 blacks as "brisk" but they did not give any figures. Results will be announced from eight to ten days after the election ends Friday, officials said.

This election is a bid by South Africa, which has ruled this territory for 58 years, to install a moderate and friently black government in the last buffer state between itself and a hostile black-ruled Africa.

Both SWAPO and the United Nations oppose the election because it is not under U.N. supervision in accordance with a Western-inspired plan adopted by the world body.

The five Western Security Council members, led by the United States, are still trying to persuade South Africa, whose League of Nations mandate to administer Namibia has been revoked by the United Nations, to accept their plan and allow a U.N. supervised election. They have said they regard this week's election as worthless.

South Africa hopes that this election will help the party it is backing to emerge as a respectable rival to SWAPO -- regarded here as a radical communist organization.

That party is the Democratic Turnhalle Alliance, a multiracial coalition of ethnically based conservative parties including the all-white Republican Party of Dirk Mudge, a former member of South Africa's ruling National Party.

South Africa hopes for a large turnout in the election so it can say the people have indicated a preference for the Turnhalle Alliance and given a rebuff to SWAPO, which is boycotting the elections and has threatened to disrupt them.

A high turnout is being aided by the use of mobile polling stations in the rural areas of this sparsely populated country.

Outside the polling stations in Windhoek, the capital, and the black township of Katutura, supporters of the Turnhalle Alliance gathered in force today, greeting potential voters with their two-fingered peace sign and assisting them in their first experience in voting.

These activities seemed to anger representatives of the all-white right-wing party called Aktur, which opposes a multiracial government.

At the two voting sites in Katutura, the Turnhalle Alliance had set up two tents larger than the balloting stations.

An angry white Aktur Party worker criticized what he called the "intimidation" of black voters by the Turnhalle Alliance workers.

Bitter fighting between the whites of the Turnhalle Alliance and Aktur has split the white Namibian population and has caused immense embarrassment to Pretoria which backed the Turnhalle Alliance because it considered Aktur too right-wing to get any international acceptance.

"We don't feel exactly betrayed," said one Aktur official, "but we do feel the dice were loaded in favor of DTA by the South African government."

Besides SWAPO, two other parties to the left of the Turnhalle Alliance have refused to participate in the elections because they are not under U.N. supervision.

Leaders in these two parties, SWAPO Democrats and the Namibia National Front, as well as influential church leaders in the territory who have worked closely with SWAPO in the past, are distressed by this election because they fear it will foreclose the possibility of an eventual U.N.-supervised election.

They are bitter at the inability of the five Western countries who drew up the U.N. plan -- the United States, France, West Germany, Britain and Canada -- to stop South Africa from holding this election.