Leaders on both sides of the widening gap between South Africa's white-minority government and its predominately black opposition reacted negatively tonight to President Reagan's sanctions measures, with whites saying the move could damage the country economically and blacks saying it does not go nearly far enough.

President Pieter W. Botha condemned the measures as "harmful" and "negative," contending they could hurt the economies of South Africa and its black-ruled neighboring states.

But Anglican Bishop Desmond Tutu, the Nobel Peace laureate, derided them as "not even a flea bite" and issued his strongest denunciation to date of Reagan, branding him a "racist."

Analysts meanwhile suggested that despite Reagan's assertion that his policy of "constructive engagement" with South Africa would continue, the sanctions may deal a final blow to the already weakened policy of warmer ties between Washington and Pretoria, leaving relations uncertain and fragile.

"Constructive engagement is terminal," said political scientist David Welsh of the University of Cape Town. "The question is what now are the alternatives, and I don't think anyone knows."

Although noting that Reagan's sanctions were "less harmful" than the bill about to pass Congress, Botha said they were "nonetheless to be regretted" because they would not speed up what he called "our reform policies" and could damage South Africa's already besieged economy.

"Whatever the intention, the effect is punitive," said Botha in a statement from Pretoria. "It is a negative step. Cooperation should not be based on coercion. Such actions diminish the ability of the United States to influence events in southern Africa."

Botha said sanctions would not force his government into changes. "Sanctions cannot solve our problems," he said. "The leaders of South Africa will themselves decide what is in our interests. Our objective is peaceful reform. Reform can only be retarded by outside attempts to interfere."

South African officials in the past generally have conceded that the sort of sanctions which Reagan signed today were of minor economic importance, but have expressed fears that they would lead to further punitive steps in the United States and Europe.

A white business spokesman, Raymond Parsons of the Association of Chambers of Commerce, reflected that view today, expressing disappointment at what he called the "unfortunate precedent" set by Reagan. Parsons said the move "stems more from political pressures in the United States than from serious defects in the policy of constructive engagement."

Officials also fear American sanctions will be a major psychological blow, isolating South Africa from the West -- a process that many believe has already begun with the refusal of western banks to extend new loans here. A ban on shipment of arms to South Africa was imposed by the United Nations in 1977.

Botha said the new measures would hurt the region economically because "even limited sanctions destroy jobs and progress." He said it was "ironical" that the sanctions were being instituted "at a time when positive steps have already been taken by the South African government to end racial discrimination."

But while he deplored the new measures, Botha stopped short of criticizing Reagan personally or issuing a sweeping condemnation of American policy. He noted the announced return of U.S. Ambassador Herman Nickel after a nearly three-month absence and said he would "withhold further comment until I have had the opportunity to study the contents" of a personal message Nickel is carrying to him from Reagan.

There was no immediate comment from Zulu Chief Gotsha Buthelezi or other black moderates who have opposed economic sanctions by the United States or others in the past.

But Tutu, widely acknowledged as the most influential black spokesman not imprisoned, bitterly criticized Reagan's measures as a face-saving device that would have little if any impact on South Africa's government.

"I'm not impressed," said Tutu in a telephone interview. "He is merely trying to save himself from the humiliation of a veto override. Your president is much more interested in helping Republicans to be reelected than in ending the bloodshed. He has no real interest in the welfare of blacks."

"The South African government is laughing all the way to the bank. They know it's not even a flea bite."

Tutu said Reagan has shown in sticking with constructive engagement and in his public statements endorsing police actions against black demonstrators a callous disregard for the estimated 675 blacks who have died since political unrest erupted here a year ago.

"He has really been saying blacks are expendable," said Tutu. "He sits around in equanimity because the fatalities are black fatalities. I said [last week in a television interview] he was a crypto-racist. I think I should say now he is a racist pure and simple."

The bishop cited recent polls suggesting strong support from urban blacks for some form of economic sanctions, even though many analysts argue such measures could sacrifice black jobs. He said the United States, if it wants to be taken seriously by blacks, should enforce the same kinds of measures against South Africa it is applying to the leftist government of Nicaragua. The Reagan administration has instituted a full trade embargo against that Central American country.

"Get rid of constructive engagement, apply to South Africa the policies you apply to Nicaragua, and voila, apartheid will end," he said.

The United Democratic Front, a broadly based antiapartheid coalition, did not comment directly on Reagan's plan but said it was further proof that the Pretoria government "is rapidly driving itself into social, political and economic exile from the rest of the world."

Botha's determination to pursue his government's tough stand against those it considers enemies, despite international disapproval, was underlined today by the arrests of four leaders of the country's campaign against conscription. At least a dozen houses and offices of the movement were raided in four cities.

No reason was given for the roundup but Adrian Vlok, deputy defense minister, in a speech yesterday charged the campaign was "being used" by the outlawed African National Congress resistance movement "to achieve the ANC's evil goals."

The South African Council of Churches condemned the arrests, noting the campaign was nonviolent and saying the government was "ruthlessly" oppressing "every single legitimate attempt to appeal peacefully for the removal of the barrier of apartheid that is destroying our country."

There were scattered incidents of unrest around South Africa today, with police reporting 45 arrests, mostly of black and mixed-race students protesting school conditions.

Police also reported arresting 140 more persons using sweeping emergency powers decreed by Botha seven weeks ago. That brings to 2,667 the number detained under the state of emergency, 905 of whom remain imprisoned without charge or access to legal counsel. One of those released during the weekend was Tutu's son Trevor, arrested Aug. 26.

Prison officials also announced that they had granted permission to the family of Nelson Mandela to pay a "special additional visit" to him this week, following reports that the imprisoned 67-year-old black nationalist leader was suffering from a kidney ailment. Earlier they had said Mandela's wife Winnie and two daughters had used up all of their allocated visits until Christmas.

In Cape Town, nearly 500,000 school children of mixed race began an enforced holiday after the closure of 450 public schools by government decree following a week of unrest in the area. The closings have provoked widespread condemnation from teacher and community organizations.