Anti-corruption investigators can hold Sarkozy for questioning for up to 24 hours, with a possible extension of another day.
Sarkozy had turned himself in for questioning a day after investigators detained his lawyer Thierry Herzog and two magistrates.
The investigators are seeking to establish if the former president, with the help of Herzog, attempted to pervert the course of justice.
They suspect Sarkozy, 59, sought to obtain inside information from one of the magistrates about the progress of another probe and that he was tipped off that his mobile phone had been tapped by judges looking into the alleged financing of his 2007 election campaign by former Libyan dictator Moamer Kadhafi.
The case could be devastating for Sarkozy's hopes of a political comeback in time for the next presidential campaign in 2017.
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