The ANC forms Umkhonto we Sizwe (The Spear of the Nation or "MK") to initiate an armed struggle. MK's policy is to target symbols of apartheid, not people. It's first sabotage attacks occur on December 16 in Durban.

Our manifesto, signed by our commander-in-chief Nelson Mandela, on December 16, 1961, read, "We are undertaking this armed struggle to show that black South Africans will not any longer live on their knees. There comes a time in the life of any people when there are but two choices: to submit or to stand and fight. The decision is to stand and fight. But at any time that the apartheid government wishes to negotiate a peaceful settlement, we would be prepared to negotiate."
—Dennis Goldberg, anti-apartheid activist and member of MK

Mandela and his colleagues move underground, raising funds, organizing and sending leaders to China to be trained in guerilla fighting. Mandela travels throughout Africa gathering support for the ANC.

Suddenly in 1961 we want to start the armed struggle again. With what? You are talking about a movement that was above ground for 40 years and every leader known.
—Joe Matthews, who accompanied Mandela throughout Africa to generate support for the ANC

Coming back from a meeting in Durban, Mandela is arrested and taken to jail. He is sentenced to five years for leaving the country illegally.

Police raid Lilliesleaf farm in Rivonia outside Johannesburg and find documents revealing MK's plans for sabotage and an armed struggle. They arrest the core of the MK leadership. Mandela, who is already in prison, is also brought to trial.

In the Rivonia Trial, eight ANC Youth League members are tried for sabotage and violent attempts to overthrow the government. Mandela's closing speech, in which he says he is fighting for a cause "for which I am prepared to die", catapults him into world attention. They are sentenced to life in prison.

When he made that speech there was dead silence in the courtroom. Nobody said anything. The judge didn't know what to say. I knew it was a moment in history.
—Dennis Goldberg, co-defendent in the Rivonia Trial.

Mandela and his colleagues are sent to Robben Island. Leaders from various political organizations are kept together in 'B Section,' allowing them to continue their political debates inside prison.

We believed strongly that we would not die in jail. That we would return.
—Nelson Mandela