* Ruling means Zuma can face 783 corruption charges
* The decision to charge Zuma lies with state prosecutor
* The charges stem from government's 1990 arms deal
* Zuma ran for the presidency after the charges were dropped (Adds Zuma asks NPA to delay indictment)
By Tanisha Heiberg
BLOEMFONTEIN, South Africa, Oct 13 (Reuters) - South Africa's Supreme Court of Appeal on Friday upheld a High Court ruling to reinstate nearly 800 corruption charges filed against Jacob Zuma before he became president.
Zuma, who has faced and denied numerous other corruption allegations since taking office, said he was disappointed by the court's decision and asked the National Prosecuting Authority (NPA) to "consider representations" before deciding whether to proceed against him.
It was unclear when the NPA would make a decision on the charges, which relate to a 30 billion rand ($2 billion) government arms deal arranged in the late 1990s.
The ruling is likely to amplify calls for Zuma, 75, to step down before his term as president ends in 2019 and diminish his influence over who succeeds him when the ruling African National Congress (ANC) chooses a new leader in December.
"It is going to make it a lot more difficult for his camp to get their presidential candidate elected," said Gary van Staden, political analyst with NKC African Economics.
Friday's unanimous Supreme Court ruling upheld a High Court decision in April 2016 that the charges against Zuma should be reinstated. They had been set aside in April 2009 by the then-head of the prosecuting authority, paving the way for Zuma to run for president later that year.
Rejecting an appeal brought by Zuma and the NPA, Judge Lorimer Leach said it was "irrational" for the NPA to have set the charges aside -- the same word used by the High Court.
"It is difficult to understand why the present regime at the NPA considered that the decision to terminate the prosecution could be defended," the Supreme Court's Leach said.
NPA spokesman Luvuyo Mfaku said prosecutors would need to consider the judgement but would "at all times do the right thing within the confines of the rule of law and in the interest of proper administration of justice".
In a separate statement, the NPA said Zuma's lawyers had asked the state prosecutor "not to serve an indictment on his client nor to re-enrol the matter prior to the representations having been considered".
The prospect that Zuma's removal may be inching closer lifted the rand against the dollar.
Zuma is unpopular with many investors after sacking respected finance minister Pravin Gordhan in March, a move that hit South African financial assets and helped tip the country's credit ratings into "junk" territory.
Zuma has faced persistent corruption allegations, most recently over leaked emails that suggest his friends the Gupta family may have used their influence to secure lucrative state contracts for their companies.
Reuters has not independently verified the emails, and Zuma and the Guptas have consistently denied any wrongdoing.
Mmusi Maimane, leader of the main opposition Democratic Alliance, said he would write to the National Director of Public Prosecutions, Shaun Abrahams, to demand he indict Zuma.
"The charges have been formulated and the evidence is ready. We now await a trial date," Maimane told reporters.
Asked by reporters whether Zuma should resign, the African National Congress (ANC) Secretary General Gwede Mantashe said the party would allow that process to take its course.
"The first thing is whether the NPA will decide to charge the president," he said. "You're putting the cart before the horse."
National prosecutor Mokotedi Mpshe's decision in 2009 to drop the 783 charges against Zuma in what has become known as the Spy Tapes case was based on phone intercepts presented by Zuma's legal team. They suggested the timing of the charges in late 2007 may have been part of a political plot to stop Zuma, then deputy president, running for the top job.
Zuma was linked to the 1990s arms deal through his former financial adviser, who was jailed for corruption.
Analysts said the credibility of Zuma and the NPA were in question after the ruling. One lawyer, Ulrich Roux, who speaks to media as a legal analyst, said "a private prosecution could be instituted" if charges are not brought. ($1 = 13.4003 rand) (Additional reporting by Mfuneko Toyana, TJ Strydom, Ed Stoddard and Alex Winning in Johannesburg; Writing by James Macharia; Editing by Catherine Evans)
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