South Africa has given the clearest signal yet of its intention to push ahead, alone if necessary, for elections leading to independence in neighboring Namibia.

The South African government announced Tuesday that it will begin registering voters Monday for election of a constituent assembly in the territory, also known as Southwest Africa. The initiative is being taken even though the main black guerrilla movement fighting for independence does not cooperate in the process.

Announcements of the three-month voter registration drive by the South African appointed administrator of the territory, Marthinus Steyn, comes amid intense diplomatic efforts by five Western powers - the United States, Canada, Britain, France and West Germany - to win acceptance of their proposals by the querrilla group, the Southwest African People's Organization.

South African has already agreed to the package hammered out by the five powers over the past 14 months in separate discussions with SWAPO and South Africa. SWAPO, with Cuban and Soviet aid, is waging a low-intensity bush war against South Africa. The white-ruled nation has administered the territory since 1920 under an old League of Nations mandate, now considered illegal by the United Nations and the World Court.

The Western plan envisages participation of SWAPO and the United Nations. If South African moved on its own, it would be creating a situation similar to that in Rhodesia - where plans call for a black government to be elected without the support of guerrilla forces. The stage is thus set there for civil war and the possibility of increased Soviet and Cuban involvement on the side of the antigovernment guerrillas.

On Namibia, South Africa is contending that it is simply following the plan worked out by the West. "We have decided to implement the Western proposals," Steyn said in announcing the voter drive. He added that the United Nations was welcome to send a representative to oversee the process.

The five Western powers, rejecting the initial South African announcement of the voter-registry plan last week, said it was inconsistent with their proposals. The Western plan would have a U.N. representative as party to any moves implementing the plan.

Under the plan, a civilian U.N. task force and South Africa would jointly organize elections for a constituent assembly, while a U.N. peacekeeping force would maintain order as South African troops withdrew from their positions.

If SWAPO fails to accept the Western proposals, they are unlikely to get U.N. approval in any case, since the United Nations has recognized SWAPO as the sole legitimate representative of the Namibian people. Without U.N. approval, there could be no peacekeeping force.

The latest Western attempt to get SWAPO to agree to the Western plan also involves an approach to South Africa for a further concession. Following a meeting in Angola 11 days ago between SWAPO and black African countries cooperating in the Western effort, the five Western powers reportedly were informed that SWAPO would accept their proposals if South Africa would agree to station its forces in southern Namibia instead of in the center of the country as the plan now stipulates.

Most observers here believe South Africa is unlikely to agree to this request, having made clear that it accepted the Western plan April 25 on the basis that it was final and not negotiable.

The launching of the voter drive is the second South African unilateral action that could jeopardize SWAPO's acceptance of the Western plan at a sensitive phase in the negotiations. On May 4, just before SWAPO was to hold talks with the five in New York, South Africa carried out a massive southern Angola, killing close to 600 people and causing SWAPO to call off the talks.

One of the main reasons that South Africa has decided to go ahead without waiting for SWAPO's acceptance apparently is the recent announcement by a SWAPO dissident leader, Andreas Shipanga, that he will return to Namibia, form his own party and participate in the elections.

Shipanga, who holds more moderate views that SWAPO's president, Sam Nujoma, was jailed for a more than a year in Tanzania and then expelled from SWAPO after Nujoma accused him of plotting an overthrow.

South Africa would appear to be hoping that Shipanga's participation in the elections will give the contest credibility if Nujoma rejects the Western plan.