South African police arrested the son of Anglican Bishop Desmond Tutu, the Nobel Peace laureate, following a courtroom incident today in which he criticized last week's police roundup of children in the black urban center of Soweto.

Police also announced the arrests of 90 more persons, bringing to 2,319 the total detained under the broad emergency powers they were granted last month, and a leading church group activist in the city of Durban was detained under the state's sweeping internal security law.

Three more deaths of black men were reported today in incidents of unrest. The official death toll since political violence started a year ago in this white minority-ruled country is now approaching 650.

Trevor Tutu, 29, the oldest of the Johannesburg bishop's four children, was arrested under South Africa's emergency decree, under which he can be held for two weeks without charge, bail or access to a lawyer or to family.

The incident reflected the growing bitterness in Soweto over last week's crackdown by white police and soldiers against hundreds of children. More than 360 have been charged with violating new antiboycott restrictions against being off school grounds during class hours.

Hundreds of parents and children packed a large hall converted into a court in Soweto today so that a number of the cases could be processed. The children were released in their parents' custody pending another hearing.

Most had been arrested last Thursday and Friday, and many had been held overnight without charge at the Moroka police station. They had been released Friday only after intervention by Bishop Tutu and other clerics calmed an angry group of parents gathered outside.

Witnesses said Trevor Tutu was observing today's hearing when the name of a small and young-looking boy was called. He then remarked, "What a shame it is for such a young boy to be arrested . . . . This is a joke."

Tutu then was taken out of the courtroom by a plainclothes white policeman to a nearby prosecutor's office, warned against making comments in court and released. A group of angry onlookers followed Tutu and gathered outside the office, accusing police of abusing emergency powers.

On his way back to the courtroom there was an exchange of words between Tutu and the policeman. He then was rearrested and charged with criminally defaming the officer.

At the Moroka station, according to Richard Spoor, Tutu's lawyer, the criminal charge was dropped, and his client was detained under the emergency regulations.

In a telephone interview tonight, Bishop Tutu, who had not been at the courtroom, said he was proud of his son. He called the arrest "part of the burden of being black in this country . . . . If it helps in a little way to focus attention on what is going on and on the abuse of police power that is taking place, it's a good thing."

Also arrested today was Paddy Kearney, director of Diakonia, a politically active church organization in Durban. Police said he was being held under Section 29 of the Internal Security Act, which allows for indefinite imprisonment in solitary confinement without charge and without access to lawyers or family.

Kearney was arrested at Diakonia's office after police ordered the Catholic archbishop of Durban, Denis Hurley, and Kearney's lawyer to leave the premises.

Hurley, who arrived with two other church officials shortly after police, said he demanded a search warrant and was told by the police that they did not need one. He was then ordered to leave.

Kearney joins 32 other activists, all supporters of the opposition United Democratic Front, a multiracial antiapartheid movement, who were arrested under the same law since Friday in Cape Town, Durban and Johannesburg.

The case against the front's founding patron, the Rev. Allan Boesak, for trying to enter a black township illegally was postponed today until Nov. 6, leaving him free to lead Wednesday's planned protest march to Pollsmoor Prison outside Cape Town. Police have warned they will take "stern action" against the illegal march, which Boesak has called to demand the release of black nationalist leader Nelson Mandela, who is serving a life sentence at the prison.