South Africa restores rights to some leading critics -- but for how long?

An effort to win some favor from Western governments has caused South Africa to end the ''twilight existence'' of a group of vociferous critics banned under this country's tough internal security laws.

For the critics, the severe restrictions on their freedom of movement, association, and speech were allowed to expire July 1. Local experts say it probably constitutes the largest single withdrawal of banning orders since the government began regularly using this draconian form of political oppression in the 1960s. The government has allowed some 55 persons - black and white, male and female - to come out from under banning restrictions, while continuing to impose them on 11 others.

Analysts identify two reasons for the government's action:

* Pretoria is confident it has internal security well in hand. The government's security concerns are focused on the externally based African National Congress. The outlawed ANC has stepped up its sabotage campaign against South Africa's white minority government, and Pretoria is concentrating on pressuring neighboring states to deny granting bases to the black nationalist group.

* The government was eager to win some favor with the West, particularly the Reagan administration. US criticism of South Africa has been muted under President Reagan, but what criticism there has been often has focused on human rights issues.

South Africans who are concerned about human rights issues were quick to point out that although they welcome this step, it does nothing to prevent future abuses. Banning orders come and go, they note, depending on the government's willingness to tolerate political dissent. As soon as dissent becomes effective, bannings or some other method of repression like detention without trial will be employed, say government critics.

For them, banning continues to constitute a basic denial of human liberties. The government can ban persons with whom it is displeased, but who have broken no law.

''Such power should not exist at all,'' says veteran member of Parliament Helen Suzman, human rights spokesman for the opposition Progressive Federal Party. ''These people are not brought to court or given an opportunity to legally defend themselves.''

For those who have come out from under banning orders, there was delight. Said Johannesburg lawyer Priscilla Jana: ''I'm so very, very relieved. It's like finding my own voice again.''

''The main thing is that I can now relax with people and not feel as if I am my own jailer,'' commented Maurice Smithers who was banned last year.

For many of those released from banning restrictions, the event was bittersweet. They know the law allows them to be banned again in the future, at the government's discretion. Some vowed to continue where they had left off before being banned. Albertina Sisulu, wife of ANC executive member Walter Sisulu, who is imprisoned in South Africa, said: ''I will intensify whatever programs of action had to be implemented when I was banned.''

Another prominent person now free of banning restrictions is Dr. Mamphela Ramphele, a former close associate of black consciousness leader Steven Biko, who died under suspicious circumstances in 1977 while in police custody. She promised to continue the community work she has been involved with both before and during her banning.

Some of the prominent persons who remain banned are Winnie Mandela, wife of imprisoned ANC leader Nelson Mandela, and Beyers Naude, an elderly white Afrikaner clergyman once prominent in the pro-government Dutch Reformed Church.