Zuma was charged Friday -- one week after he defeated Mbeki in the party leadership vote -- with corruption, racketeering, fraud and money-laundering, the Financial Times reported Monday.
Zuma's allies accuse Mbeki of pressuring the National Prosecuting Authority to file the charges.
The Congress of South African Trade Unions said the timing of the charges had "all the hallmarks of vengeance, deep-seated anger and frustration by the NPA and whoever else is behind this."
However, Mokeketedi Mpshe, the acting director of public prosecutions, said Sunday that the charges against Zuma have nothing to do with politics.
"It's got nothing to do with the president," he said. "It's absolute nonsense."