South Africa's top court has 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 Monday 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.

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.

'Irrational judgement'

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.

"This judgement is irrational," he said.