The inquest into the death in political detention of white labor union activist Neil Aggett opened today with an effort by government and police lawyers to prevent a sworn statement by Aggett alleging police mistreatment from being submitted as evidence.

When Magistrate Pieter A. Kotze ruled that the statement, made 14 hours before Aggett's death, could be used, police lawyer Piet Schabort applied to appeal the decision before the Supreme Court of Transvaal Province, and his application was granted.

Aggett, 27, a doctor who gave up medicine to work for a predominantly black trade union, was found dead in his cell Feb. 5 after having been held by security police without charges since Nov. 27.

In another dramatic move as the inquest began today, security police arrived at the Johannesburg magistrate's court and served a banning order on Maurice P. Smithers, who had been in detention with Aggett and smuggled a note out saying he had seen him being maltreated by his interrogators.

The power of South Africa's security police and how they use it is one of the country's most emotive political issues. At least 46 prisoners have died in detention, and many others have alleged torture.

Four years ago, the death in custody of black consciousness leader Steve Biko caused an international outcry and a surge of anger among the black population. Revelations of police ill-treatment at the Biko inquest shocked many of the government's own white Afrikaner supporters and, shaken by the repercussions, Pretoria instructed the security police to exercise greater care in handling political prisoners.

Aggett was the first political detainee to die since then, and his death already has had a radicalizing effect on many of the country's new black labor unions.

It has also served to bring the nonracialist, but militant African National Congress to the fore again, while the more racially exclusivist black consciousness movement espoused by Biko may be going into decline.

Smithers' smuggled note about Aggett was raised in Parliament by Helen Suzman, the veteran civil rights campaigner and opposition politician, who refused to name him at the time.

According to Suzman, Smithers claimed to have watched through a window while Aggett was beaten by his interrogators and made to do exhausting exercises in the nude.

Shortly before the police arrived at the court, where Smithers was a spectator, the lawyer for Aggett's family, George Bizos, announced that he intended to call him as a witness.

The two-year banning order prevents Smithers from moving outside the Johannesburg area, from being quoted, or from being in the company of more than one person at a time.

He will still be able to give evidence, however.

The effect of the Supreme Court's decision on the police appeal will go beyond the issue of whether Aggett's allegations are admissible.

It will determine the scope of the whole inquest: whether it can probe the security police treatment of Aggett, or whether it must confine itself to the clinical facts of what caused his death.

The police, who deny charges of mistreating him, want a narrow hearing, while Bizos argued today that the charges are of central importance to the case.

The family does not concede the police contention that Aggett had hanged himself with a piece of cloth, Bizos said, "but if it was suicide we will argue that it was induced suicide."

Anything throwing light on how he was treated, or on his state of mind only 14 hours before his death, was relevant, the lawyer argued.

Earlier the district surgeon (a state-employed doctor) of Johannesburg, Vernon Kemp, who conducted an autopsy on Aggett, described the injuries he found.

In addition to abrasions around the neck where the cloth had been tied, he itemized several small scars and recent injuries on Aggett's back and face.

Aggett's funeral on Feb. 13 turned into a massive joint demonstration by black unionists and the African National Congress, which is committed to trying to overthrow white-minority government by force.

Although the reaction to Aggett's death has not reached the level it did over Biko, the authorities seem anxious to contain it.