South Africa took a major step toward implementing its own settlement in the territory of Namibia (Southwest Africa) today when it formally inaugurated a multiracial national assembly in which its Sovietbacked guerilla opponents are not represented.

The move, widely regarded by Western observers as a unilateral act leading toward limited independence in South African terms for the mineral-rich country, will make it more difficult for the West to revive a U.N. plan for independence. That plan would have provided for guerilla participation and a cease-fire in the 13-year-old guerilla war.

It will also complicate Western efforts to ward off African governments' call for economic sanctions on South Africa. These calls are expected in the next few weeks at the United Nations.

The assembly's first meeting today took place during a massive deployment of South African troops to the northern part of the territory. Some reports say the deployment includes up to 10,000 reservists recently called up from South Africa. The operation appears aimed at capturing guerillas of the Southwest Africa Peoples' Organization (SWAPO) who killed four whites and two blacks in separate incidents two weeks ago.

In a television last night, South African Foreign Minister Pik Botha, indicated he saw little hope of breaking the deadlock in negotiations over two clauses holding up implementation of the U.N. plan and described the situation as "a farce."

Botha accused the five Western countries involved in drawing up the U.N. plan - the United States, Britain, Fance, West Germany and Canada - of "deceit and double-crossing" and said, "As far as I'm concerned, how can one negotiate with deceit? I'm not prepared to do it."

A spokesman in Botha's office said later, "The ball is now in the West court. If they want to repair the damage, they have to rectify the two clauses" to which South Africa objects. One clause provides for the establishment of SWAPO bases in Namibia during the cease-fire and another permits SWAPO bases in neighboring Angola to be excluded from monitoring by U.N. troops.

The military operation illustrates some of the difficulties for the South Africans in the Namibian conflict if the U.N. plan is shelved.

The guerilla group has eluded South African troops for more than two weeks. The military today announced the shooting over the weekend of two civilians in the town of Tsumeb, near where the other slayings took place.

Meanwhile at camps in Angola, more SWAPO recruits reportedly are being trained by East German and Cuban advisers.

The new national assembly will be dominated by the Democratic Turnhalle Alliance, a coalition of ethnically based parties allied to the South African government and believed by many to be financed by it.

Among its first legislative actions, the assembly is expected to make it impossible for the political arm of SWAPO to continue to operate in Namibia.

The assembly is also likely to move quickly to end racial discrimination in public places to boost its popularity among the country's estimated 850,000 blacks.

Meantime, a white-dominated party that up to now had boycotted all South African unlaterial moves in Namibia, has announced it is ready to join the national assembly. The Federal Party said it was "now of the opinion that the [U.N. plan] would not be implemented in the immediate future," and it was prepared to participate in the South African-sponsored national assembly "as an interim measure."

In doing so, the Federal Party is breaking ranks with its closest political ally, the black-dominated Southwest African Peoples' Union, exemplifying the splits along racial lines now taking place in the territory.