Items including the late anti-apartheid leader's ID card and some of his iconic shirts have been listed for sale by a US auction house working with Mandela's daughter Makaziwe.
But South Africa's Ministry of Culture now says it has filed an appeal to halt "the unpermitted export" of the objects.
"Former president Nelson Mandela is integral to South Africa's heritage," Minister Zizi Kodwa said. "It is thus important that we [...] ensure that his life's work and experiences remain in the country for generations to come."
Auction house Guernsey's had described the sale -- expected to fetch several million dollars -- as "remarkable" and "unprecedented."
"To imagine actually owning an artifact touched by this great leader is almost unthinkable," Guernsey's wrote on its website.
But the house's president, Arlan Ettinger, told AFP the government's move, which follows an earlier unsuccessful bid to halt the sale, put the firm "in the very, very difficult position of saying; 'Do we go forward with the auction?'"
A black silk jacquard shirt Mandela wore when he met Britain's Queen Elizabeth in 1996 is advertised with a starting price of $34,000 while an ostrich leather briefcase is going for a minimum of $24,000.
Offers for his 1993 personal identification booklet are to start at $75,000.
Guernsey's said Mandela's eldest daughter, Makaziwe, authorized the event as a fund-raiser for a memorial garden to be built next to her father's resting place in the village of Qunu.
Initially advertised in 2021, the auction was first suspended a year later after the South African Heritage Resources Agency (SAHRA) went to court arguing it included items of historical and cultural importance.
South African judges gave it the green light in December, but SAHRA and the culture ministry have since lodged an application for leave to appeal. A decision is pending. SAHRA spokesman Ben Mwasinga told AFP any item associated with a former statesman requires an export permit to be sold abroad, which has not been granted.
Objects of particular concern included Mandela's shirts and writings, he added. Ettinger said the authority's position was "misguided."
"It would be hard for anyone to define these [items] as being culturally or historically significant," he said.
He described the artifacts as a "small collection of personal items" in some cases given by a father to his daughter that would have likely ended up "on shelves in a closet somewhere and be forgotten in time" rather than in a museum.
The auction house was to consult with its attorneys and the Mandela family to decide the next steps, he added.
Mandela died in 2013 aged 95. The auction was supposed to take place at the Lincoln Center in New York City and online on February 22.