Why was Nelson Mandela imprisoned?

Nelson Mandela was imprisoned as a terrorist in 1962 and would remain behind bars for the next twenty-seven years.
At the time of his imprisonment, the organization that he led—the African National Congress (ANC)—was banned by the apartheid government. The government had brought in very strict laws against any kind of opposition to apartheid, and the ANC was considered the most dangerous opponents of all, not least because they attracted the most support.
The ANC regarded themselves as freedom fighters, but to South Africa's white minority government they were a terrorist organization engaged in armed subversion against the state. To be sure, Mandela and other members of the ANC did engage in acts of sabotage against the South African state, such as bombing military installations, bridges, power plants, and telephone lines. The idea was to cause the maximum amount of damage to the apartheid state with minimal casualties.
Irrespective of the ANC's intentions, however, the government saw such actions as a threat to the very existence of the apartheid state and responded accordingly. By imprisoning Mandela and other senior members of the organization, they hoped to break the back of the ANC, but things didn't work out quite like that. From inside his prison cell, Mandela became an icon of resistance whose name became famous the world over as a symbol of the anti-apartheid struggle.