Former Brazilian leader Luiz Inacio Lula da Silva, a top contender to win next year's presidential election, has been convicted on corruption charges on Wednesday and sentenced to nearly 10 years in prison.

The ruling marked a stunning fall for Lula, one of the country's most popular politicians, and a serious blow to his chances of a political comeback. The former union leader, who won global praise for policies to reduce stinging inequality in Brazil, faces four more corruption trials and will remain free on appeal.

The verdict represented the highest-profile conviction yet in a sweeping corruption investigation that for over three years has rattled Brazil, revealing a sprawling system of graft at top levels of business and government.

Judge Sergio Moro found Lula, 71, guilty of accepting 3.7 million reais ($1.2 million) worth of bribes from engineering firm OAS SA, the amount of money prosecutors said the company spent refurbishing a beach apartment for Lula in return for his help winning contracts with state oil company Petroleo Brasileiro

Federal prosecutors have accused Lula, Brazil's first working-class president from 2003 to 2011, of masterminding a long-running corruption scheme that was uncovered in a probe into kickbacks around Petrobras.

Lula's legal team said in an emailed statement that he was innocent and they would appeal.

"For over three years, Lula has been subject to a politically motivated investigation," they wrote. "No credible evidence of guilt has been produced, and overwhelming proof of his innocence blatantly ignored."

Lula's lawyer Cristiano Martins has repeatedly accused judge Moro of being biased against his client, which Moro strongly denies.