The nations of the world professing nonalignment condemned South Africa today for its decision to install a "puppet regime" in Namibia and urged the U.N. Security Council to take action to block the "sinister scheme."

Foreign ministers of the 80-nation Coordinating Bureau of the Nonaligned Movement, meeting here in a special session on Namibia, scrapped their planned opening-day agenda in light of Pretoria's moves yesterday to establish an "internal administration" in Namibia. They drafted a strongly worded statement at the suggestion of Indian Prime Minister Rajiv Gandhi, chairman of the movement.

In a speech that set the tone for the nonaligned group's declaration to the Security Council, Gandhi said South Africa's steps to set up a transitional government in Namibia with a group of parties called the Multiparty Conference were intended to "fabricate possibilities of unilateral declaration of independence on the part of this puppet body." Gandhi called the move "a brazen affront to the Namibian people . . . and indeed to the entire international community."

The nonaligned ministers' declaration said the South African move constitutes "the most brazen defiance of the United Nations," which had declared in resolutions that such a step would be regarded as illegal and had called for withdrawal of South African troops in Namibia and the holding of U.N.- supervised elections.

"The movement of nonaligned countries once again reiterates its firm and unwavering commitment to the heroic struggle of the Namibian people for freedom and national independence, under the leadership of the South-West Africa People's Organization SWAPO , the sole and authentic representative of the Namibian people," the statement said.

SWAPO, in a statement issued in Lusaka, Zambia, criticized South Africa, according to Reuter, saying that the independence fight had "weakened the South African colonial regime's resolve to fight, forcing it to attempt to turn our people's struggle . . . into a civil war. This is the whole objective of the so-called interim government."

[U.N. Secretary General Javier Perez de Cuellar issued a statement condemning the plan after a meeting with South Africa's ambassador, news services reported. Perez de Cuellar told Ambassador Kurt von Schirnding that the United Nations regards Namibia's new interim government as "null and void."]

Gandhi, seated with SWAPO President Sam Nujoma, Palestine Liberation Organization Chairman Yasser Arafat and other nonaligned delegates, criticized western nations that, he said, are tacitly encouraging South African intransigence on the Namibian issue.