The Reagan administration yesterday condemned as a "serious mistake" the South African government's decision to declare a national state of emergency and to arrest scores of black opposition leaders, saying such "repressive measures" showed a lack of understanding about "the fundamental causes of unrest and violence there."

The administration took no immediate concrete action, however, to show its displeasure. It conveyed the U.S. view through diplomatic channels to the South African government in Pretoria.

White House spokesman Larry Speakes also said there would be no change in the administration's strong opposition to imposing economic sanctions on the white-ruled South African government.

"There are no plans to change our policy and our viewpoint regarding sanctions," Speakes said. "We oppose sanctions. They hurt those we are trying to help."

State Department spokesman Bernard Kalb called sanctions "a blunt instrument" that would "not promote the U.S. goal in South Africa" and would serve only to encourage intransigence rather than moderation by white and black leaders.

Administration spokesmen left open the possibility that the United States might take diplomatic or other actions in the next few days to show its displeasure. On May 23, the United States expelled the senior South African military attache here, Brig. Alexander Potgieter, in response to South Africa's bombing of black nationalist offices in three black states. A year ago, the United States recalled its ambassador to South Africa, Herman Nickel, following similar South African action.

The administration, Speakes added, calls on both sides to use restraint "to avoid violence, to enter into a dialogue, to work out a negotiated settlement to the problems that exist in South Africa."

At the same time, Kalb said the administration was "extremely disappointed" that the so-called Eminent Persons Group (EPG), a seven-man mediation team appointed by the Commonwealth countries, has decided to end its efforts to find a basis for talks between South Africa's white and black leaders.

The group yesterday made public a report concluding that the Pretoria government was "not ready yet to negotiate fundamental change," and accused it of "obstinacy and intransigence." It suggested that economic sanctions were necessary to avert "what could be the worst bloodbath since the Second World War."

"We believe the best response by the South African government to the pessimistic conclusions of the EPG report would be to accelerate steps toward ending apartheid," Kalb said. "It must clearly demonstrate its readiness to negotiate with real [black] leaders."