Court finds Charles Taylor guilty of aiding rebels; sentencing set for May NEW: The "conviction brings some measure of justice," an activist says NEW: Taylor has been a pivotal figure in Liberian politics for decades (CNN) -- An international tribunal found former Liberian President Charles Taylor guilty Thursday of aiding and abetting rebels who raped, killed and mutilated civilians in neighboring Sierra Leone, the first verdict against an African head of state in a world court. However, prosecutors failed to prove he had command over the rebels, said Justice Richard Lussick of the Special Court for Sierra Leone. A stoic Taylor -- dressed in a charcoal gray suit , a white shirt and a burgundy tie -- stood quietly as the judge delivered the guilty verdict. Lussick said prosecutors proved beyond reasonable doubt that Taylor is "criminally responsible" for aiding and abetting crimes during the brutal civil war, including murder, rape, sexual slavery and conscripting children under the age of 15. The former warlord pleaded not guilty to 11 charges of war crimes, crimes against humanity and other international violations. A sentence hearing is set for May 16, with the sentence to be issued two weeks later. United Nations human rights chief Navi Pillay described the conviction as "immensely significant," saying it sends out a message that even the most powerful are not above the law. "It is important to recognize that Taylor may appeal the verdict, and that his guilt is not fully established until the end of the judicial process," Pillay said. Nevertheless, whatever the final outcome, this is undoubtedly a historic moment in the development of international justice. A former president, who once wielded immense influence in a neighboring country where tens of thousands of people were killed, mutilated, raped, robbed and repeatedly displaced for years on end, has been arrested, tried in a fair and thorough international procedure. His support for rebels fueled the bloody war, prosecutors said. Witnesses testified about grisly violence by the rebels during the conflict, including chopping off the arms of civilians, and shooting and disemboweling pregnant women and children. "Brave men, women and children have taken the stand against Charles Taylor," the prosecutor's office said in a written statement. Fighters included teenagers forced to kill, rape and plunder under the influence of drugs to provoke violent behavior. Prosecutors said Taylor financed the war with proceeds from the so-called "blood diamonds" used to fund rebels in several African conflict areas. Taylor was president of Liberia for six years until 2003, when heavy international pressure forced him out of office. Officials set up the court to avoid instability in Sierra Leone, where emotions about the civil war still run high. "While today's conviction brings some measure of justice to the people of Sierra Leone, Taylor and the others sentenced by the Special Court are just the tip of the iceberg," said Brima Abdulai Sheriff, director of Amnesty International in Sierra Leone. Thousands of persons suspected of criminal responsibility for incidences of unlawful killings, rape and sexual violence, mutilations and the use of children in Sierra Leone's armed conflict have never been investigated, much less prosecuted. Campbell told the special tribunal that she received "dirty-looking stones" she assumed were gifts from Taylor after a dinner hosted by then-South African President Nelson Mandela in 1997. They gave me a pouch and said, 'A gift for you,'" she said.