The South African government is dropping all orders for the ''preventive detention'' of key political opponents. The latest move comes amid strong criticism in the United States of this country's system of racial segregation and the harsh security laws that help sustain it.
An official South African source said Monday's action was in response to an improved domestic situation after three months of black unrest in South Africa. But it is the second time in four days that Pretoria has released from jail some of its opponents. Many analysts feel the moves are related to mounting criticism in the US of South Africa and what some see as the Reagan administration's too-soft approach in dealing with Pretoria.
The latest measures are seen on the one hand as attempts by South Africa to, in effect, reward the Reagan administration for sticking with its policy of so-called ''constructive engagement,'' which includes an approach of quiet diplomacy that Pretoria prefers to more public criticism.
Yet the whole issue of the Reagan administration's effectiveness in bringing change to South Africa has taken on importance because of the swell of public demonstrations across the US against South Africa. Indirectly at least, Pretoria's actions appear to be a response to those demonstrations.
Critics consider preventive detention one of the most flagrant violations of human rights in South Africa. Security laws here allow the police to detain people without trial for several purposes. Preventive detention is permitted if the South African minister of law and order fears the person may commit an offense or endangers the security of the state or the maintenance of law and order.
According to the Detainees' Parents Support Committee, a group that monitors detentions, 11 people were being held by the South African police under preventive detention. The government had also served preventive detention orders on at least four others who were not in police custody. They include Archie Gumede, William Nairn, and Paul David - government opponents who have been holed up in the British consulate in Durban since Sept. 13.
The preventive detention orders against all these people have been withdrawn. But a police spokesman said that while some of these would go free, some others would now be formally charged with a crime. (United Press International reports six Asian leaders technically freed from political detention were immediately rearrested and charged with treason.)
Most of the people being held or sought under the preventative detention statute were active in a political campaign last August that urged Colored (persons of mixed-race descent) and Indian voters to boycott elections for the government's new tricameral Parliament. The new Parliament for the first time included Colored and Indian participation. Blacks remain excluded.
Last Friday a separate group of 11 detainees was released. These were trade union and community leaders involved in a black workers' strike in November in protest against the government's handling of black unrest. It was also announced on Friday that another five detainees connected with the strike were being charged with subversion. Their trial will take place early next year. In the meantime they have been released on bail.
Analysts here say it seemed more than coincidence that the release last Friday occurred on the same day that South African Nobel Peace Prize-winner Bishop Desmond Tutu met with President Reagan. Monday's move came on the same day as Bishop Tutu picked up the peace prize in Norway. Bishop Tutu is a staunch opponent of the South African government and a critic of the Reagan approach to South Africa.
Human rights advocates say the South African government has become increasingly repressive this year, as a result of protests against its new Parliament and serious black unrest that has claimed some 150 lives. The Detainees' Parents Support Committee says more than 1,000 people have been detained without trial this year, the highest number for ''many years.''
Pretoria objects to this statistic, saying there have been only 434 detentions in South Africa. The discrep-ancy lies in the fact that the remainder have taken place in the so-called independent black ''homelands.''
Pretoria denies responsibility for these territories, but the parents committee believes these areas remain part of South Africa.
Whatever the numbers, human rights advocates object to the system that permits detention without trial for an indefinite period. They advocate the scrapping of the laws that make such detention possible.