AFPSouth Africa's Deputy President Cyril Ramaphosa has described the sacking of Finance Minister Pravin Gordhan as "totally unacceptable".
His late night dismissal on Thursday led to a 5% plunge in the value of the currency, the rand.
Mr Gordhan was seen as a bulwark against corruption in an administration that is facing growing criticism.
He was one of several victims as President Jacob Zuma conducted a major overhaul of his cabinet.
Mr Zuma said the midnight reshuffle was about a "radical socio-economic transformation".
Mr Ramaphosa told state broadcaster SABC that he would not resign in response to the sacking but continue to " serve the people".
SOUTH AFRICA GOVERNMENTIn a spirited farewell press conference at the treasury's office in Pretoria, Mr Gordhan defended his economic record, and dismissed speculation by that he had recently held meetings in London aimed at undermining Mr Zuma.
Malusi Gigaba, previously the Minister of Home Affairs, has replaced Mr Gordhan.
Sfiso Buthelezi will become Deputy Finance Minister, replacing Mcebisi Jonas.
REUTERSPresident Zuma also made a number of other changes in the cabinet.
"I have directed the new ministers and deputy ministers to work tirelessly with their colleagues... to ensure that the promise of a better life for the poor and the working class becomes a reality," President Zuma's statement said.
Earlier this week, President Zuma recalled Mr Gordhan from planned events in the UK.
Last October, Mr Gordhan was charged with fraud - but the charges were later dropped.
He has described the allegations as politically motivated.
'This is about politics' - Alastair Leithead, BBC News
AFPPresident Jacob Zuma said his reshuffle was all about a "radical transformation" but by sacking detractors and hiring loyalists it was clearly about politics.
Earlier this week, Mr Gordhan was in London reassuring investors to have confidence in his country's economy, but his sacking will have the opposite effect.
His replacement is a man with little economic experience but a keen supporter of the president.
There will be a sharp economic and political reaction to this move by a president still fighting off allegations of corruption and what is known as "state capture", over the political influence of a controversial family.
A nuclear power station deal with Russia worth billions, could now be back on, and commentators will wonder if radical transformation refers to his recent comments about taking land without compensation to give to the people.
Mr Gordhan had been seen as standing up to President Zuma in cabinet and has warned against corruption becoming rampant.
The South African Communist Party, an ally of the governing African National Congress, had earlier lodged a formal objection to plans to dismiss Mr Gordhan, who is widely respected internationally.
Many senior ANC figures also opposed the finance minister's removal.
Meanwhile, the Democratic Alliance, South Africa's main opposition party, said it would call a vote of no-confidence in President Zuma in parliament.
Pressure has been growing on Mr Zuma to step down amid numerous allegations of corruption.
He has been accused of allowing members of the Indian-born Gupta family to wield undue influence in his administration in a scandal known as "state capture".
The replaced Deputy Finance minister Mcebisi Jonas alleged that he had been offered the job of finance minister by one of them.
Mr Zuma denies the claims.