Erdogan has said he is battling "a state within a state" and described the corruption probe, which comes ahead of crucial March polls, as a smear operation.
Media reports yesterday said prosecutors had begun handing out corruption indictments to some of the 89 suspects arrested three days earlier, with the first eight formally arrested and placed in pre-trial detention.
They are accused of numerous offences including accepting and facilitating bribes for development projects and securing construction permits for protected areas.
The crisis erupted Tuesday when police made the arrests in a series of dawn raids, one of the most brazen challenges to Erdogan's 10-year rule.
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Among the suspects detained were the sons of Interior Minister Muammer Guler, Economy Minister Zafer Caglayan and Environment Minister Erdogan Bayraktar, along with the chief executive of state-owned Halkbank, Suleyman Aslan, and construction tycoon Ali Agaoglu.
The environment minister's son has since been released, but the other two ministers' sons are still being held for questioning, the Dogan news agency reported.
The crisis has rattled the stock market and sent the Turkish lira to an all-time low.
Since the scandal broke out, Erdogan has sacked dozens of police officials, including the Istanbul police chief, for cooperating with the investigation without permission.
Turkish media said another 17 were fired on Friday alone, amid a widening purge of the police command.
Erdogan's critics accuse him of desperately trying to protect his cronies, and the appointment of Selami Altinok, a little-known governor with no police career, as Istanbul's new police chief was further seen as an attempt to shut down the investigation.