A South African court is due to decide if a corruption case can proceed against ruling party leader Jacob Zuma.
The African National Congress (ANC) head is facing 16 charges relating to a multi-billion dollar arms deal.
Mr Zuma, favourite to become president in next year's elections, says the charges are politically motivated and wants them to be dismissed.
Hundreds of people are outside the court in Pietermaritzburg to support Mr Zuma after an all-night vigil.
Police have closed roads and put up cordons to prevent a fresh outbreak of violence, following clashes in the city of Durban earlier this week.
Supporters of Mr Zuma have repeatedly made threats of unrest, and some have said they are prepared to kill for the ANC leader.
The BBC's Mpho Lakaje in Pietermartizburg said the crowd who braved the very cold wintery and drizzly night outside the court was singing and chanting in Zulu "vuli ndlela", which loosely translated, means open the door.
Their message was that the Supreme Court should leave the way open for Mr Zuma to become president next year by dropping the case, our reporter says.
"We are here to show him he is not alone today," one supporter told the BBC.
"Often these judges are political judges and I feel so sad in my heart with Jacob because I love him and I need him to be the president next year," he said.
Mr Zuma, aged 66, is due to attend the hearing in Pietermaritzburg later on Friday.
The charges relate to the arms deal in 1999, when the South African government announced its largest post-apartheid arms deal, signing contracts totalling 30bn rand ($5bn; £2.5bn) to modernise its national defence force.
The deal involved companies from Germany, Italy, Sweden, Britain, France and South Africa.
Mr Zuma has said he will stand down as ANC leader only if he is found guilty of the charges - corruption, fraud, racketeering and money-laundering.
His colleagues in the leadership of the ANC are standing firmly by him.
They say he is the victim of a political conspiracy intended to prevent him becoming South Africa's president in elections due to be held before July 2009.
Mr Zuma has strong support from the youth league and trade unions
Tension was raised this week, following the publication of a cartoon showing Mr Zuma about to rape a figure labelled "Lady Justice" - a reference to the criticism of judges by Mr Zuma's supporters.
"We will eliminate any forces that come our way," ANC Youth League president Julius Malema said on Wednesday, according to local media.
In 2005, Mr Zuma was sacked as South Africa's deputy president when his financial adviser was found guilty of soliciting a bribe on behalf of Mr Zuma and jailed for 15 years in connection with the arms deal.
He then went on trial, but the case collapsed in 2006 when the prosecution said it was not ready to proceed.
He was charged again last December shortly after winning a bitter campaign against President Thabo Mbeki to become ANC leader.