The 34-year-old 'Batman Begins' star, who was in South Africa with her seven-year-old daughter Suri for the filming of 'The Giver', was emotional on visiting Robben Island, where Mandela was held as a political prisoner.
Holmes was given guided tour by Mandela's former fellow inmate Ahmed Kathrada and he told Hello magazine that Holmes was choked with emotion when she witnessed the harsh conditions endured by convicts.
"Katie was very touched by the whole experience. We visited Mandela's cell, where I told her that we all slept on the ground for 14 years, that there were no flush toilets, only buckets, that the lights were kept on all night... She didn't cry, but she did react," Ahmed said.
"You could see she was very emotional," he added. The actress later penned a message to Ahmed in the prison's visitors' book, "My sincerest gratitude for your work, your struggle, your triumph, your dedication, and your inspiration."