Inkatha, which was involved in an increasingly violent struggle with the ANC. Mandela requested talks with Botha but was denied, instead secretly meeting with Minister of Justice Kobie Coetsee in 1987, and having a further 11 meetings over the next three years. Coetsee organised negotiations between Mandela and a team of four government figures starting in May 1988; the team agreed to the release of political prisoners and the legalisation of the ANC on the condition that they permanently renounce violence, break links with the Communist Party, and not insist on majority rule. Mandela rejected these conditions, insisting that the ANC would end its armed activities only when the government renounced violence. Mandela's 70th birthday in July 1988 attracted international attention, including a tribute concert at London's Wembley Stadium that was televised and watched by an estimated 200 million viewers. Although presented globally as a heroic figure, he faced personal problems when ANC leaders informed him that Winnie had set herself up as head of a gang, the "Mandela United Football Club," which had been responsible for torturing and killing opponents—including children—in Soweto. Though some encouraged him to divorce her, he decided to remain loyal until she was found guilty by trial. Victor Verster Prison and release: 1988–1990 Recovering from tuberculosis exacerbated by the damp conditions in his cell, Mandela was moved to Victor Verster Prison, near Paarl, in December 1988. He was housed in the relative comfort of a warder's house with a personal cook, and he used the time to complete his LLB degree. While there, he was permitted many visitors and organised secret communications with exiled ANC leader Oliver Tambo. In 1989, Botha suffered a stroke; although he would retain the state presidency, he stepped down as leader of the National Party, to be replaced by F. W. de Klerk. In a surprise move, Botha invited Mandela to a meeting over tea in July 1989, an invitation Mandela considered genial. Botha was replaced as state president by de Klerk six weeks later; the new president believed that apartheid was unsustainable and released a number of ANC prisoners. Following the fall of the Berlin Wall in November 1989, de Klerk called his cabinet together to debate legalising the ANC and freeing Mandela. Although some were deeply opposed to his plans, de Klerk met with Mandela in December to discuss the situation, a meeting both men considered friendly, before