Three black American jurists visiting South Africa, including a federal judge, were detained and interrogated for two hours by security police in the tribal homeland of Ciskei, the visitors told reporters tonight.

The three were among five American jurists invited here by a South African black lawyers' association. The five said tonight that they were also tailed by men they believed to be members of the security police throughout a two-day visit to Cape Town and questioned at a roadblock when they visited a pass-law court there.

U.S. District Court Judge Thelton Henderson, of San Francisco, said at Johannesburg Airport tonight before flying home that he found the experiences distressing but felt that they had helped him understand better what it is like to be a black South African.

"It is considerably different from being a black American," he said.

The encounter with the Ciskei security police took place Aug. 11, the Americans said. The chief of the homeland's security police, Maj. Gen. Charles Sebe, flatly denied today that it ever occurred.

Questioned about the incident after the American group had described it to South African newspapers and to the U.S. Embassy in Pretoria, Sebe said: "This never happened. It is a bunch of lies."

Sebe is a brother of President Lennox Sebe of the homeland, one of several areas reserved for blacks that South Africa has granted nominal independence.

Henderson said he, civil rights lawyer Julius Chambers, of Charlotte, N.C., and Vivian Chambers wanted to visit a courthouse in the town of Zwelitsha, where a group of young blacks was appearing in a security trial.

When they arrived, two plainclothesmen ordered them to go to the headquarters of the Ciskei Central Intelligence Services.

A lieutenant colonel and a full colonel of the intelligence service questioned them, the Americans said.

"They told us they had information we were spies," Henderson said. "They went on and on about the dangers of communism and how the Communists use other people as spies without them being aware they are being used."

They were released after about two hours.

The judge also described how the group was tailed for two days by two white men, whom they took to be security policeman, during a visit to Cape Town.

When the group -- which also included a circuit judge of the U.S. Court of Appeals, Leon Higginbotham Jr., of Philadelphia -- visited a court in an African township where blacks are tried under the pass laws that control their movements about the country, they were pulled aside for questioning at a roadblock, members of the group said.