Shun Chetty, a lawyer who defended South African black dissidents including the late Steve Biko, has fled South Africa to neighboring Botswana and is seeking asylum in Britain, a lawyer's group in Washington said yesterday.

Millard Arnold, director of the Southern Africa Project of the Lawyers' Committee for Civil Rights Under Law, said Chetty feared that South African authorities were about to "ban" him and prevent him from practicing law.

Banning is used by South African authorities mainly to silence and control dissidents critical of the country's apartheld racial policy. Under the system, anyone declared to be "banned" is forbidden to meet with more than one person at a time, speak publicly or publish articles.

Chetty, who is of Indian origin, achieved prominence as a civil rights lawyer when he defended leaders of a black organization against charges of terrorism during a 1975-76 trial. The defendants, members of the South African Students' Organization - Black People's Convention, had passed out pamphlets and given lectures on changing South Africa's white-dominated system, Arnold said. They were each sentenced to five to six years in prison after what he called a "show trial."

Chetty is best-known as the lawyer for the family of Steve Biko, who died while under arrest on Sept. 12, 1977. The South African government recently decided to pay $78,000 to Biko's family as compensation for his death.

Upon learning of Biko's death in a Pretoria prison, Chetty immediately ordered a family doctor to assist at the government-sponsored post mortem that found Biko's fatal injuries were due to a blow to his head. Chetty represented the Biko family at the inquest.

In another important trial, Chetty organized the defense for 11 black high school students charged with sedition for their part in the demonstrations that sparked the upheavals in Soweto in 1976.

Although all the students were found guilty, only four received jail terms. Seven others were given suspended sentences.

More recently Chetty was involved in the case of 12 members of the African National Congress, one of two banned black nationalist groups. They have been charged with high treason and 43 counts of terrorism. Chetty was to have defended them at their trial, which was set to begin shortly.

Arnold said Chetty called him from Botswana yesterday to say that he and his wife had fled South Africa and hoped to visit the United States after moving to Britain, where he is seeking asylum. Botswana already has granted asylum to Chetty, who reportedly does not have a passport, Arnold said;.

Chetty was reluctant to immediately discuss details of his recent activities in South Africa or what prompted him to leave the country, Arnold said.

He said he suspected that Chetty's decision to leave stemmed from continued harassment by South African authorities and a recent interrogation by one of the country's law associations.

Arnold said Chetty was summoned in mid-May by the Transvaal Law Society, a regional bar association, and questioned for four hours about his practice, his finances, his reasons for concentrating on political trials and his relationship with foreign groups.

"It wasn't the questions that were harassing, but the way the session took place," Arnold said. "The interrogation was of a most sinister nature."

Arnold said Chetty told him last month that he believed the session was the first step toward having him banned.

Chetty "felt he would be prevented from practicing law and that if this happened he would be of no value in South Africa and might as well leave," Arnold said.

Chetty had never been detained for his activities as far as is known, Arnold said, although his house and office had been raided. Arnold said Chetty had been "subjected to various forms of pressure" amounting to a "Chinese water torture type of harassment."

For example, Arnold said, South African security police have accosted clients leaving Chetty's office and warned them to find other attorneys.

Arnold said other clients whom Chetty was representing had been summoned to their trials without Chetty's knowledge and had been defended by court-appointed lawyers.

His involvement with mainly political trials and the fact that much of the money used to defend the disidents came from overseas organizations opposed to apartheid made Chetty a controversial figure. He is greatly admired by most blacks, but intensely disliked by the government and right-wing whites.

His work drew him steady surveillance by the security police both at his office in the Oriental Plaza shopping center - a mall segregated for Indian traders only - and at his home in an Indian neighborhood.

Earlier this year, a progovernment daily, the Citizen, ran a front-page story linking Chetty romantically with U.S. Foreign Service officer Sally Shelton, who is not ambassador to Barbados.

Shelton has acknowledged their friendship but has denied any romatic involvement. The front-page treatment of the story for two days was widely seen here as an attempt by the paper - which has close ties to the security police - to embarrass and discredit Chetty.