Former French president Nicolas Sarkozy is accusing the French justice system of trying to humiliate and destroy him.
Authorities placed Sarkozy under formal investigation Wednesday - one step below charges - in connection with alleged corruption.
Judges had ordered him to be taken into custody Tuesday. He faced nearly 16 hours of questioning. High-ranking French politicians are rarely placed in custody.
Sarkozy said in a television interview that the action was intended to humiliate him, and that the French justice system is being used for political purposes.
He said he has never betrayed the confidence of the French people or committed any act that violates the rule of law.
Authorities accuse the ex-president of using his influence to block an investigation into his 2007 election campaign, in which he is alleged to have accepted funds from then-Libyan dictator Moammar Gadhafi.
The conservative Sarkozy lost the 2012 election to Socialist Francois Hollande. He has been hoping to make a political comeback.