Nelson Mandela opened his law office in 1952, and a few months later teamed up with Tambo to create the first Black legal practice in South Africa. It was difficult for both Mandela and Tambo to find time for both their legal practice and their political aspirations. That year Mandela became president of the Transvaal ANC, but was banned under the Suppression of Communism Act – he was prohibited from holding office within the ANC, banned from attending ANY meetings, and restricted to the district around Johannesburg.
Fearing for the future of the ANC, Nelson Mandela and Oliver Tambo initiated the M-plan (M for Mandela). The ANC would be broken down into cells so that it could continue to operate, if necessary, underground. Under the banning order, Mandela was restricted from attending meeting, but he drove down to Kliptown in June 1955 to be part of the Congress of the People; and by keeping to the shadows and the periphery of the crowd, Mandela watched as the Freedom Charter was adopted by all the groups involved. His increasing involvement in the anti-Apartheid struggle, however, caused problems for his marriage and in December that year Evelyn left him, citing irreconcilable differences.
On 5 December 1956, in response to the adoption of the Freedom Charter at the Congress of the People, the Apartheid government in South Africa arrested a total 156 people, including Chief Albert Luthuli (president of the ANC) and Nelson Mandela. This was almost the entire executive of the African National Congress (ANC), Congress of Democrats, South African Indian Congress, Coloured People's Congress, and the South African Congress of Trade Unions (collectively known as the Congress Alliance). They were charged with "high treason and a countrywide conspiracy to use violence to overthrow the present government and replace it with a communist state." The punishment for high treason was death. The Treason Trial dragged on, until Mandela and his 29 remaining co-accused were finally acquitted in March 1961. During the Treason Trial Nelson Mandela met and married his second wife, Nomzamo Winnie Madikizela.
The 1955 Congress of the People and its moderate stance against the policies of the Apartheid government eventually led to the younger, more radical members of the ANC to break away: the Pan Africanist Congress, PAC, was formed in 1959 under the leadership of Robert Sobukwe. The ANC and PAC became instant rivals, especially in the townships. This rivalry came to a head when the PAC rushed ahead of ANC plans to hold mass protests against the pass laws. On 21 March 1960 at least 180 black Africans were injured and 69 killed when the South African police opened fire on approximately demonstrators at Sharpeville.
Both the ANC and PAC responded in 1961 by setting up military wings. Nelson Mandela, in what was a radical departure from ANC policy, was instrumental in creating the ANC group: Umkhonto we Sizwe (Spear of the Nation, MK), and Mandela became the MK's first commander. Both the ANC and PAC were banned by the South African government under the Unlawful Organisations Act in 1961. The MK, and the PAC's Poqo, responded by commencing with campaigns of sabotage.
In 1962 Nelson Mandela was smuggled out of South Africa. He first attended and addressed the conference of African nationalist leaders, the Pan-African Freedom Movement, in Addis Ababa. From there he went to Algeria to undergo guerrilla training, and then flew to London to catch up with Oliver Tambo (and also to meet members of the British parliamentary opposition). On his return to South Africa, Mandela was arrested and sentenced to five years for "incitement and illegally leaving the country".