Johannesburg — Under mounting political pressure, President Reagan appears to have changed the style, if not necessarily the substance, of his policy toward South Africa. Yet diplomats and political analysts here see that shift as significant, given the importance of ''style'' and its frequent inseparability from the ''substance'' of how foreign nations deal with the prickly problem of South Africa.
In a speech Monday marking the 36th anniversary of the Universal Declaration of Human Rights, President Reagan abandoned his usual approach of quiet diplomacy and strongly attacked South Africa. He said he felt a ''moral responsibility to speak out'' against the ''human and spiritual costs of apartheid. . . .''
Senior United States officials reportedly confirmed that the speech meant the Reagan administration was going to turn up the volume of criticism of South Africa's policy of racial segregation.
(Nobel Prize-winner Desmond Tutu, a black South African bishop, claimed some credit for Reagan's tougher criticism of Pretoria. ''Reagan must have disliked my calling his policy of 'constructive engagement' with South Africa evil, immoral, and unchristian,'' he told Reuters.)
The harsh criticism from President Reagan followed several weeks of mounting public and political pressure in the US for a tougher US approach to South Africa. Pretoria has in recent days made some minor concessions - the release from detention of about 27 political opponents, 11 of whom who have since been charged with subversion and treason - that appeared aimed at trying to defuse some of the US criticism.
But at the same time Pretoria has been adamant that it is not bending to overseas pressure. South African Foreign Minister Roelof Botha said the recent release of some government opponents was not the result of prescriptions by foreign governments and ''still less by demonstrations and radical action by pressure groups.''
Analysts here are skeptical that the substance of the Reagan policy will change much. One diplomat says the US policy, in essence, is not much different from that practiced by Britain and West Germany. But he adds that the nuances of the US approach are important, because the US has more clout with South Africa than any other Western nation.
The problem for all Western nations is how to push a reluctant Pretoria toward change when outside pressure, particularly highly visible pressure, often forces the country's whites to dig in their heels.
Simply washing its hands of South Africa is not a practical option for the West. It has sizable investment in South Africa, is dependent on certain important minerals produced here, and is worried about leaving the field of southern Africa open to the machinations of the Soviet Union.
''The great debate is how you handle apartheid. We all attack it but how do you change it?'' asks one diplomat. He adds: ''Anyone who looks at this problem can see that you need a combination of carrot and stick. Endless stick simply doesn't work.''
The signals coming from the Reagan administration indicate that it will be using a little more stick - in the form of public criticism - than it did in the last four years. In practice this may mean that criticisms conveyed in the past in private to Pretoria will now be more open.
In his speech, President Reagan specifically criticized South Africa's policies of detention without trial and the forced removals of blacks from white areas to the so-called ''homelands.''
One European diplomat said developments in the US were probably of ''profound significance'' to South Africa, although he doubted Pretoria would take immediate action to ease US criticism.
''External pressure does not have all that much effect on (South Africa),'' said the diplomat, ''particularly when it creeps out in the public domain.''
Indeed, that has been a basic assumption behind the Reagan administration's preference for using quiet diplomacy. Chester Crocker, US assistant secretary of state for African affairs and the main architect of so-called ''constructive engagement,'' has practiced quiet diplomacy in the belief that strong, public attacks only make Pretoria more intransigent.
But analysts here believe the political pressure on the Reagan administration simply grew too strong, forcing the President to become more outspoken.
''Constructive engagement is working on the external scene,'' says one Western diplomat. He says the US had helped defuse some regional tensions in southern Africa by encouraging more dialogue between South Africa and arch-enemies like Mozambique (which signed an accord with Pretoria earlier this year) and Angola (from where South Africa has agreed to withdraw its military forces).
Yet this and other Western diplomats here say that ''constructive engagement'' has been largely a failure in bringing domestic change in South Africa. The past three months have seen widespread black unrest in South Africa and mounting detentions of political opponents.