Johannesburg — Each Thursday morning a lone figure carrying a placard stands in protest outside the Rand Supreme Court here.
The object of the protest - mounted solo because open-air political gatherings are banned in this country - is South Africa's practice of detaining people in a manner that sets it apart from other Western nations. In South Africa, detainees can be held indefinitely with no charges being brought, and they have no legal right to consult with a lawyer.
The practice took an international turn recently with the detention earlier this month of British citizen Steven Kitson while on a trip to South Africa to visit his father - a convicted saboteur imprisoned here 17 years ago.
The Kitson detention brought British protest and new criticism in South Africa of the practice of detention. There is a renewed push here to abolish or amend security laws that allow detention for an indefinite period without charges.
Some South African legal experts also are looking overseas - particularly to the United States - for new international pressure Pretoria to curb detention practices.
''This is an area where quiet diplomacy has failed,'' says South African legal expert John Dugard, referring to President Reagan's policy of ''constructive engagement'' toward South Africa.
Mr. Dugard, director of the Center for Applied Legal Studies at the University of the Witwatersrand, believes the US approach of using ''carrots'' instead of ''sticks'' to encourage change has clearly backfired here in the field of human rights.
He says South Africa is sensitive to international criticism. But Pretoria's confidence that the US will not react strongly to detentions has removed an ''important measure of restraint,'' Mr. Dugard asserts.
The South African Institute of Race Relations calculates that ''at least'' 620 people were detained in 1981. In 1982, police have been detaining people at a rate of about one per day.
Keeping track of detentions is no easy matter. The government does not make official reports about detentions, although at the request of a member of Parliament the government disclosed earlier this month that it is holding 159 people in detention.
For the parent or relative of a detainee, lack of information about the individual can be the most painful part of the experience.
The police are not obliged to notify the family when a relative is detained. Generally they report a detention only when queried by the family or a lawyer. ''You just don't know anything. That is what is so terrible,'' says the mother of a young man detained in late October.
Most detentions occur under two legal provisions: Under Section 22 of the General Laws Amendment Act, a person can be held without charge for 14 days. Under the so-called Section 6 of the Terrorism Act, a period of detention can be indefinite.
Little is known about the conditions of detention. South African law provides for visits by magistrates to those detained, but the Black Sash women's organization says it is not clear that the magistrates are accountable to anyone. Black Sash has voiced concern that if the magistrates report to the security police, detainees would be reluctant to make complaints.
''The situation in South Africa warrants extraordinary security steps,'' Mr. Dugard says. Indeed, guerrilla attacks in this country are on the rise and are increasingly sophisticated.
However, he asserts, South Africa has ''overreacted.'' Its heavy hand in internal security will create a backlash by ''undermining respect for the legal process, particularly among blacks,'' he says. He notes the powers to detain individuals are more sweeping here than in Northern Ireland.
The numbers of detentions, as well as the methods, are of concern here. In 1976-77, after rioting in Soweto, there was a high number of detentions. After that era there was a marked drop-off. But detentions in 1980 and 1981 are at a level more than twice the numbers of the previous two years.
University students and labor union officials seem to be the largest categories of those detained last year. The apparent crackdown on labor officials has raised concern that the government is deliberately undercutting ''reforms'' in the labor field.
In Johannesburg a support committee made up of detainees' parents formed to try to improve conditions of detention and to push for security-law changes.
Parliament is slated take up the issue this year after a government-appointed commission completes an investigation.