Julius Malema was key to getting Mr Zuma into power
The head of South Africa's powerful ANC youth wing has hit out at his critics, who have accused him of "exploiting the poor" and leading a lavish lifestyle.
Julius Malema, a left-wing firebrand and ally of President Jacob Zuma, has been accused of making 130m rand (£11m; $17m) from state contracts since 2008.
But he says he resigned from his firms in 2008 and accused politicians and the media of a "smear campaign".
He challenged his critics to bring criminal charges if they suspected him.
Mr Malema's support was a crucial factor in securing Mr Zuma's rise to leadership of the ruling African National Congress (ANC).
The BBC's Pumza Fihlani in Johannesburg says Mr Malema has always touted himself as someone who understands poverty, because his mother was a single parent who worked as a cleaning lady in rural Limpopo.
But our correspondent says the 28-year-old's apparent wealth will no doubt compromise his message to his supporters - many of whom are poor.
Poverty remains rife in the country and many South Africans are losing patience with the ANC.
Violent protests have raged in recent months with many complaining that the government has failed to provide basic services such as clean water, electricity and proper housing.
Mr Malema was throw into the spotlight by media reports on the weekend claiming that SGL Engineering Projects, a company he reportedly holds a stake in, had won millions of dollars in government contracts to provide services.
He reportedly used some of his earnings to buy two lavish homes and three luxury cars, including an Aston Martin.
But Mr Malema denies being a millionaire and says he has done nothing wrong.
He told a news conference on Monday that he did not owe anyone an explanation about the state of his finances.
He said he had resigned directorships in his firms in May 2008, after he was appointed as leader of the ANC's youth wing.
"I instructed my lawyers to process my resignations from all the corporations and companies I was involved in when I was based in Limpopo," he said.
In later radio interviews, he admitted that he did not actually sign papers legalising the processes and did not follow up to see if the lawyer had acted.