Minister in the Presidency Jackson Mthembu died of Covid-19-related complications on Thursday, just 10 days after testing positive for the disease.
President Cyril Ramaphosa described him as "an exemplary leader, an activist and life-long champion of freedom and democracy".
"He was a much-loved and greatly respected colleague and comrade, whose passing leaves our nation at a loss," Ramaphosa said in a statement.
Mthembu, 61, served as chief whip of the ANC in the National Assembly from 2016 to 2019.
Politicians responded with shock and sadness to the news of his passing.
ANC deputy secretary-general Jessie Duarte said she was still in shock and the news hasn't sunk in as yet.
"I've known him from the UDF days," she said. "He was one of the most hardworking people. We feel a great sense of loss as the ANC. He was one of the most experienced, genuine activists that I knew and an experienced person in managing stakeholder relations with all manner of people."
She said he was particularly good in his last portfolio as minister in the presidency, and "he leaves a gap that won't easily be filled".
Duarte said one of the moments she would treasure was the way he smiled the day Nelson Mandela was released from prison.
'The delight on his smile'
He was the UDF organiser in the then Eastern Transvaal and part of the media propaganda machine there.
"And the delight on his smile, that's one of the things I think about," she said.
ANC chairperson and Mineral Resources and Energy Minister Gwede Mantashe said: "It's a big loss to the country, it's a big loss to the ANC. He was a reliable person in the movement. He was reliable in the work of government, we send condolences to his family."
Mantashe said he knew Mthembu was ill and he phoned him regularly.
"I was thinking of phoning him this morning. Last I phoned him was three days ago and ... I had all the hope, by the way. He talked to me nicely, a short message, and we left it there. I can't understand it."
'He was my NEC companion'
Human Settlements Minister Lindiwe Sisulu said: "I can't find the words. He has always been a very good friend, jovial, open and genuine in his feeling and committed to the ANC. I've known him since I came back from exile and we have always huddled together at NEC, and gossiped and teased each other. He was my NEC companion."
Herman Mashaba posted a tweet, sending condolences. "We must not allow politics to forget our humanity. Giants are falling to this disease. May he rest in peace."
DA leader John Steenhuisen, who served as the chief whip of the opposition, opposite Mthembu, said: "I knew him as a man of integrity, and someone who managed to see the bigger picture and the greater cause. He was always prepared to do what it took to find solutions to whatever impasse we might have been facing in the House. If this meant meeting up long after the working day was done to thrash out the details of an agreement, then Jackson would do so in a heartbeat."
'Standing strong on principle'
FF Plus' Corne Mulder tweeted: "Jackson Mthembu was an excellent chief whip of Parliament. He stood strong on principle when Parliament came under attack during the Zuma years. Later, he was a competent and dedicated member of the executive. His death is a tragedy for us all."
UDM leader Bantu Holomisa served in the Portfolio Committee on Environmental Affairs when Mthembu chaired the committee; he said Mthembu always came well-prepared to the meetings.
"Jackson, when confronted with ANC politics, appeared to be a person who would stand up for his beliefs openly and had a good sense of humour. Mthembu batted well during his innings and we can't fault him. We should be just thankful to have people like him, who were in the forefront of the struggle."