Neil Aggett, the white labor union secretary found dead in a security police cell in Johannesburg Friday, had been detained in what appears to be a major crackdown on the activist wing of South Africa's growing black labor union movement.

Aggett, a 27-year-old doctor, was one of 17 persons closely connected with the activist unions who were detained in a series of security police raids Nov. 27. Many others had their homes and offices searched.

A total of 306 persons active in black labor unions were arrested last year.

Louis le Grange, the minister of police, told Parliament Wednesday that many of the detainees would be brought to court soon in a major trial "involving a very serious matter."

Le Grange said the detained unionists were not being held only because of their labor union activities.

"We detain them because they are directly involved in threatening internal security and especially because they are involved in alleged African National Congress activities," he said.

The African National Congress, biggest of the African nationalist parties, is outlawed in South Africa. It is trying to overthrow white minority rule through guerrilla action by its military arm, Umkhonto Wa Sizwe (Spear of the Nation).

Aggett's friends and colleagues strongly deny that he was a member of the congress and that he was working under cover of the labor movement.

They describe him as a committed labor unionist who gave up full-time practice as a doctor to devote himself to helping the black labor movement.

Aggett was found hanged in his cell, and the government ruled his death a suicide. Union colleagues and family members have expressed doubt that Aggett would commit suicide, describing him as a very mature and stable person.

He was secretary of one of the biggest black labor organizations, the Food and Canning Workers' Union.

Only in recent years have blacks been allowed to join and form labor unions in South Africa.

Unions are supposed to operate within a circumscribed system, in which they register with the Department of Labor and may strike only if negotiations and arbitration procedures fail.

Most black unions have refused to register, preferring direct negotiations and strike action. This is not illegal, although the government disapproves.

What it dislikes even more are activist unions that, apart from dealing with labor matters, try to gain better living conditions for blacks.

The unions call these efforts "community action." They try to mobilize pressure through employers and other activities, such as organizing consumer boycotts.

Because blacks have no vote and their main political parties are banned, they have been trying to make gains through the one channel open to them--labor unions.

The government now appears to be moving against this.

The canners' union, strongly committed to "community action," is closely linked with two others, the General Workers' Union and the South African Allied Workers' Union.

The security police action appears to be aimed mainly at these three. Both the president of the Allied Workers' Union, Thozamile Gqweta, and the vice president, Sisa Njikelana, were detained for the sixth time Nov. 27, the same day Aggett was arrested. The Allied Workers' Union secretary, Sam Kikane, also was detained that day.