This was according to a statement on Wednesday by the Ahmed Kathrada Foundation's Director, Neeshan Balton. Kathrada was hospitalised on March 4 for surgery related to blood clotting on the brain.
Balton added that the 87-year-old had "experienced several health-related setbacks making the prognosis of his illness unpredictable. He remains in hospital with everything being done to assist his recovery".
President Jacob Zuma on Tuesday had also extended well wishes to Kathrada, while the African National Congress (ANC) offered theirs on Wednesday.
Visits from members of the public are still not permitted due to hospital regulations and the necessity for a restful period of recovery. "We have been overwhelmed by the immense support, both from within South Africa and abroad. We continue asking for your prayers during this period, both for Mr Kathrada and his family and friends," said Balton.
"We have also had requests from people about where they can leave cards bearing messages of support, or flowers. This can be dropped off at the Foundation's offices (Signet Terrace Office Park, Block B, Suite 2 – 19 Guinea-Fowl Street, Ext 1, Lenasia).
"All of the messages that we have received will be collated and passed on to the family," said Balton.
Kathrada is one of just three Rivonia trialists still alive today, the other two being Andrew Mlangeni and Denis Goldberg.
Along with Nelson Mandela, Walter Sisulu, Govan Mbeki, Raymond Mhlaba, and Elias Motsoaledi, they were sentenced in 1964 to life imprisonment on Robben Island during the famous Rivonia Trial.
Meanwhile, board member of the Ahmed Kathrada Foundation, Professor Achille Mbembe, will deliver a lecture on Ahmed Kathrada and his lifelong activism against racism on Thursday.
He will speak at the unveiling and dedication of an auditorium at the Management College of Southern Africa (Mancosa) in Durban, in honour of Kathrada.
The event is being hosted during Anti-Racism Week (March 14-21), an initiative spearheaded by the Anti-Racism Network South Africa (ARNSA).