South Africa's top court on Monday barred former president Jacob Zuma from running in general elections next week, but his upstart opposition party vowed to fight on to return the graft-tainted politician to office.

The ruling has stoked fears of violent unrest in the run-up to the poll, already the most competitive since the advent of democracy in 1994.

But Zuma's party, uMkhonto Wesizwe (MK), urged supporters to remain calm and go out and vote on May 29.

"This heavily flawed and conflicted judgement is not the end but rather a pivotal moment affirming that the MK Party is the right choice for the black poor and downtrodden," it said.

The top court backed an electoral commission decision that Zuma's previous conviction for contempt of court prevents him from becoming an MP, ruling that the constitution bars anyone sentenced to more than 12 months in jail.

- 'Not eligible' -

Zuma, who left office in 2018 dogged by corruption allegations, was convicted of contempt in 2021 and sentenced to 15 months.

Rioting after his imprisonment left more than 350 people dead.

He eventually served less than three months in jail, but the court ruled that this was irrelevant as the constitution refers to the sentence imposed, not served.

"Mr Zuma... is accordingly not eligible to be a member of and not qualified to stand for election to the national assembly," Justice Leona Theron said, reading the judgement.

In a South African general election, the president is chosen by MPs from among their own ranks.

If Zuma is not allowed to enter parliament he cannot become president, even if his newly formed party is able to muster enough seats to propose him.

The electoral commission said ballot papers would not be reprinted nine days from the vote and Zuma's photo would still appear next to his party's name.

"Jacob Zuma is still the party leader, Jacob Zuma is on the ballot paper of MK party and people are still going to vote Jacob Zuma in numbers," MK secretary general Sihle Ngubane told AFP.

A few dozen supporters of the firebrand ex-leader turned opposition challenger -- some sporting the military fatigues often worn by MK members -- chanted and danced as they left the court in Johannesburg after the hearing.

"We are resolute," MK party member Lindiwe Mtshali, 39, told AFP, adding she was however "very disappointed" at the ruling.

MK has attempted to portray the decision as a desperate effort from its opponents to derail its electoral hopes.

The party boasts it will secure a two-third majority in the National Assembly that would allow it to change the constitution and reinstate Zuma -- but that seems highly unlikely.

Opinion polls suggest MK will not do well outside of Zuma's native KwaZulu-Natal province. An Ipsos survey last month estimated support at 8.4 percent nationwide.

"The country is still in shackles," prisoner rights activist and Zuma supporter Golden Miles Bhudu declared, as he protested the verdict outside the court, his arms and legs bound by chains.

"This judgement is irrational," he said.

- 'Threat of violence' -

Since leaving office, Zuma, who was president between 2009 and 2018, an era that for many South Africans has become synonymous with official corruption, has fought several legal battles.

He assumed control of MK to challenge his successor Cyril Ramaphosa's African National Congress (ANC), which has won every election since the country became a democracy but is struggling in the polls and risks losing its absolute majority for the first time.

Outside the court, Neeshan Balton, director of the Ahmed Kathrada Foundation, which had joined the case to provide legal advice as amicus curiae, welcomed the ruling.

"The former president clearly was not going to uphold the constitution, and we are happy that he's disqualified," he said.

If Zuma's outsider campaign cuts into the ANC's traditional support base, Ramaphosa may be forced to negotiate a coalition with one or more of the many small opposition parties to ensure he is re-elected.

Political analyst Sandile Swana said that Zuma's ineligibility was not going to cost MK many votes.

"What matters is that he is still able to campaign for the MK and be the face of it," he said.

The ANC was the leading political force in the struggle against the apartheid regime and many older South Africans remain loyal to it.

But support has slumped amid corruption allegations and soaring crime and unemployment rates. Just under a third of the working age population is unemployed and the murder rate has reached 84 a day.

Acknowledging the ruling, Ramaphosa said security forces were ready to deal with "any threat of violence".

© Agence France-Presse