HARARE, ZIMBABWE, SEPT. 26 -- Blinking into glaring television lights and bewildered almost into speechlessness, 11-year-old William Modibedi haltingly told his story of being tortured in a South African prison last year.

Many of the 500 delegates here for an international conference on children and apartheid gasped and some cried as Modibedi tried to recount through a translator his story of how he was picked up by security police on Oct. 6 and held for two months and two days in dank jail cells near Krugersdorp, north of Johannesburg.

His thin voice quavering and his head barely bearly rising above the table on a dais in the cavernous Harare convention center, Modibedi -- with his mother at his side trying to help him overcome his fear -- said that two of his teeth were knocked out in a beating by his interrogators and that he was given electrical shocks to force him into confessing that he had set automobiles on fire during demonstrations.

A diminutive 13-year-old girl identified as Tabisa Mabusa, who is paralyzed from the waist down, sat in a wheel chair and told the delegates that South African police agents burst into her home in Gaborone, Botswana, on June 14, 1985, and shot her in the spine as she tried to flee.

Television camera crews from Europe and the United States converged on her as she described how her aunt was shot dead in her bedroom during the cross-border raid, and how she is unable to play basketball with her friends.

A parade of other witnesses, most of them teen-agers, offered similar testimony during the three-day International Conference on Children, Repression and the Law in Apartheid South Africa, which ended today.

The conference was unique in that it was the biggest gathering yet of blacks and whites from South Africa and senior officials of the African National Congress who are living in exile in southern African states and in Europe.

But the meeting also dramatically underscored the importance of a key weapon in the ANC's public relations arsenal: the effect on children of the struggle against apartheid in South Africa.

"Nothing draws the heartstrings like the plight of children who are detained and tortured in South African prisons," said a top ANC official, speaking with the understanding that he would not be identified.

He added, "I listened to that {testimony} and I cried -- and I've seen it before. Imagine how it affects people who have never heard it before."

As a tactic in the battle with South Africa for world opinion, the issue of detained children has no match, antiapartheid campaigners here readily conceded.

This conference, they said, has been particularly important because it comes practically on the eve of two potential milestones in the worldwide campaign to intensify economic sanctions against the Pretoria government.

One is a meeting of Commonwealth heads of state in Vancouver, Canada, next month to discuss sanctions.

The second is a U.S. State Department report to Congress on the question of South Africa's response to sanctions imposed last year.

While the organizers of the conference said that the convergence of these events with the spine-chilling testimony by allegedly tortured children this week was a coincidence, they made no attempt to hide their satisfaction with the discomfort the South African government might feel at so crucial a time.

"We just hope that these people who are still not prepared to put pressure on South Africa would be made to feel ashamed of supporting a regime like this," said the Rev. Frank Chicane, secretary general of the Southern African Council of Churches.

ANC president Oliver Tambo, after hearing the childrens' testimony about alleged torture, declared, "Let all those in the West who still treat this regime as legitimate explain why they continue thus to aid and abet the commission of a crime against humanity.

"Let them stand accused as those who, by refusing to impose sanctions, extended a helping hand to the apartheid regime so that it could continue its terror against the children," he said.

The emotional nature of the subject and its potential for swinging world opinion in favor of tighter sanctions against South Africa permeated the conference.

At its outset, Anglican Archbishop Trevor Huddleston, a veteran antiapartheid campaigner, urged scores of foreign journalists covering the conference to report in vivid detail what he described as the "inhumane brutality against children in South African jails.

"Sanctions obviously are exceedingly decisive. Great Britain and West Germany will do everything possible to prevent effective sanctions. The only way open is mandatory sanctions," Huddleston said.

British antiapartheid activist Glenys Kinnock, who conducted an emotional interview with some of the child witnesses outside the conference hall for a British television news broadcast, acknowledged using the opportunity as a tactic in the battle to increase sanctions.

Vowing to take the children's testimony back to the British Parliament, Kinnock, the wife of British Labor Party leader Neil Kinnock, said, "Those who speak of the suffering that they say sanctions would cause must be reminded of what is happening daily to thousands of children in South Africa because that regime is not subject to rigorous pressure from outside."

The South African government already has shown its sensitivity to the potential damage the conference could cause to its efforts to project a new image of reform-mindedness and conciliation toward militant black nationalists.

On Wednesday, the opening day of the conference, Law and Order Minister Adriaan Vlok issued a statement in Cape Town saying that he anticipated that misleading statistics would be issued here by the Johannesburg-based Detainees' Parents Support Committee (DPSC).

Vlok said that only 115 children under the age of 18 are still being detained: three 15-year-olds, 28 aged 16 and 84 aged 17. Most of them, he said, are being held for serious crimes such as public violence and even murder.

The DPSC maintains that at least 400 children under the age of 18 still are being held among a total of 1,800 people detained without charge.

Vlok accused the DPSC of "serving the interest of the enemies of South Africa."

Several delegates from South Africa said they feared recriminations when they return home, including possible seizure of their passports.