On 1 June 1970, Nelson Mandela wrote a letter to his young daughters Zeni and Zindzi from his prison cell on Robben Island. “My darlings,” he wrote. “It is more than eight years since I last saw you …”

The girls’ mother Winnie had also been imprisoned the previous year, and their father had no idea, he wrote, who was looking after them in the school holidays, who fed them, bought them clothing or paid their school fees. He had written two letters to the pre-teenage girls the previous year, he told them, but had learned neither had reached them.

And yet he would send another, he wrote, because “the mere fact of writing down my thoughts and expressing my feelings gives me a measure of pleasure and satisfaction. It’s some means of passing on to you my warmest love and good wishes, and tends to calm down the shooting pains that hit me whenever I think of you.”

He signed it: “Tons of love, Daddy.”

Facebook Twitter Pinterest Mandela’s letter to his daughters, 1969. Photograph: Martin Godwin/The Guardian

This letter, or more accurately the modest notebook into which the prisoner himself made copies of all of his outgoing correspondence, forms part of a major new interactive exhibition about Mandela’s life and legacy that opens on Friday in London, before embarking on a global tour.

Mandela: The Official Exhibition traces the former South African president’s extraordinary life from his early years in a small farming community in the remote Transkei region, to his education and political activism, his arduous imprisonment, long-awaited release and late-in-life roles as his nation’s first post-apartheid president and global peacemaker.

Among the 150 artefacts and personal items on display are the rough sisal mat on which Mandela slept during his incarceration on Robben Island, the master key to his cell, the trenchcoat he acquired on leaving prison and wore frequently after his release, and the white lion skin which was laid, with a flag of South Africa, on his coffin after he died in 2013.

Chief Nkosi Zwelivelile (Mandla) Mandela, the former president’s oldest grandson and the exhibition’s co-curator, said he hoped the retelling of his grandfather’s life would serve as “an inspiration to humanity”.

“My grandfather dedicated 67 years of his life to serving humanity. He was a champion for justice, peace and human rights. And this exhibition is exactly about that. It is a call to action to the global community to uphold [Mandela’s clan name] Madiba’s values and principles. It cannot be that we honour him in statues and awards and not fully follow in his footsteps in what he stood for.”

Chief Mandela is the son of Mandela’s second son Makgatho by his first wife, Evelyn Ntoko Mase. Another letter on display was written by the imprisoned Nelson to Makgatho, consoling him on the death of Thembi, Mandela’s oldest son, who was killed in a road accident in 1969. “I have spent a lot of time these past few months thinking of you,” Mandela wrote. Though Makgatho had visited him three months earlier, the prisoner wrote, “it seems as if I had not seen you for a decade, so badly do I miss you”.

Facebook Twitter Pinterest Chief Nkosi Zwelivelile Mandela: ‘My grandfather said that if he has any regrets to his commitment to the struggle for liberation it was the sacrifice of his family.’ Photograph: Leon Neal/Getty Images

“My grandfather said that if he has any regrets to his commitment to the struggle for liberation it was the sacrifice of his family,” his grandson told the Guardian. “His absence and not being able to raise his kids, not being able to be there for his family, was quite a huge challenge for him and the biggest sacrifice ever.”

He said one personal favourite among the items on display was the Patek Philippe watch Mandela wore every day as president, which he always set to South African time regardless of where he was travelling. “My grandfather was always very cautious of time,” he said. “If he set a meeting for 10 o’clock he was there 15 minutes early.”

Zelda la Grange, who worked for Mandela as his personal assistant and later manager of his private office for 19 years during and after his presidency, has also provided items for the exhibition, including medical notes, pages of his diary showing his intense travelling schedule, and his United Nations identity card.

She said she hoped the exhibition would “serve as a very important reminder of where Nelson Mandela came from, and our country, South Africa, this miracle nation, what we have been through. A reminder for South Africans … [and] for people across the globe to learn from our experiences. The oppression and the injustices were man-made, and we are heading in that direction again across the world – so we have to prevent that from happening.”

She said while many of the items she had shared were very ephemeral, such as a boarding pass used by Mandela to fly to London in 1990, she considered them priceless. “To me, it’s an affirmation, because it’s unbelievable even to me that I was part of this story.”