More than 500 black churchgoers attending a service June 16 were arrested en masse by South African security police and remain jailed at undisclosed locations, Amnesty International said yesterday.

The London office of the human rights organization said in a statement it had confirmed that police with dogs had interrupted the service at the Apostolic Faith Church in Graaff-Reinet in Cape province and loaded about 600 blacks into vans.

Amnesty said that some elderly and very young members of the congregation were later freed but that more than 500 were incarcerated. Those jailed included two ministers, the Rev. Sam Shoeu of the Methodist Church of Southern Africa and the Rev. Themba Nyatyowa of the Dutch Reformed Church, the group said.

A State Department official said yesterday he was unable to confirm the arrests.

The detainees have not been allowed visitors and no details of their whereabouts have been released, Amnesty said. Services commemorating the 10th anniversary of the Soweto uprising, such as the one held in Graaff-Reinet, had been banned earlier by the government.

Members of U.S. church, union and human rights groups attempting to track detainees said in interviews this week that they were groping for accurate information. Many said they still were unsure of the accuracy of the often cited figure of 3,000 persons who may have been arrested since the state of emergency was decreed in South Africa on June 12.

Pretoria has refused to give the number or identities of detainees, and journalists in South Africa face fines, jail or possible expulsion for attempting to report those details.

The State Department, which said earlier that four of five American detainees have been released, yesterday identified the fifth American as Charles R. Zechman, 50, of Lewisburg, Pa., a missionary who has lived in South Africa since 1981.

A spokeswoman said that Zechman has been interviewed by a U.S. consular official and that a second visit was scheduled for today. Zechman, who is imprisoned at Nylstroom, two hours north of Pretoria, is being investigated for alleged violations of the Public Safety Act of 1953, the spokeswoman said.

Nana Mahomo of the AFL-CIO here said the organization, through contacts with groups like Britain's Trades Union Congress and other sources he declined to identify, has compiled a confirmed list of 98 union officers detained, including some national labor leaders. He added, however, that he believed the total number was higher.

A Washington-based rights group, the Lawyers Committee for Civil Rights Under Law, which monitors rights abuses in Africa and elsewhere, said it has between 400 and 500 names of persons it knows have been arrested.

"From the information reaching us, the numbers are increasing," Mahomo said. "Either the people are now locating where the detainees are, or the arrests are continuing. One would presume that since some of the leaders went underground, they the police may be catching up with some."

Concerned groups said they have been unable to determine the conditions under which detainees are being held, or whether previous practices involving alleged beatings and torture by security police have been continued.

Ralston Deffenbaugh of Lutheran World Ministries in New York said at least two Lutheran clergymen have been detained in Cape province. He also said that a meeting at a Lutheran diocesan office in northern Transvaal province last week was surrounded by troops in armored personnel carriers called "hippos." The same office was raided the evening of June 13 by security officers, who made no arrests but took church files and documents, he said.

The gathering of accurate information has grown more difficult as sources inside South Africa dry up, either through arrests or because of fear of violating the emergency regulations, church and rights group spokesmen said.

"I've been in touch with people over the phone, but even in private people have declined to be explicit," said Willis Logan of the National Council of Churches in New York. Logan said he had talked last week to a prominent journalist in South Africa and "we could only talk in code."

Malcolm Smart of Amnesty's London office said in a telephone interview yesterday that it had taken eight days for news of the mass arrests at Graaff-Reinet to reach Cape Town, where Amnesty first learned of them.

Smart said he talked yesterday to one white man in the Graaff-Reinet area who acknowledged that he had heard something about the arrests through his black domestic servant. "It brings home the peculiar nature of apartheid," Smart said. "It's quite dramatic that people don't know, and don't seem to care, what's occurred so close by them."

Charlie Gofen of The Washington Post Foreign Service contributed to this report.