Mr Sarkozy is being held at Nanterre, near Paris, in an unprecedented step against a former president.
Mr Sarkozy is being questioned about whether he sought inside information from a judge concerning an investigation into campaign funding.
The latest developments are seen as a blow to Mr Sarkozy's attempts to challenge for the presidency in 2017.
Investigators are trying to find out whether Mr Sarkozy, 59, who was president from 2007 to 2012, had promised a prestigious role in Monaco to a high-ranking judge, Gilbert Azibert, in exchange for information about an investigation into alleged illegal campaign funding.
They are looking into claims that Mr Sarkozy was warned his phone was being bugged as part of the funding probe.
Mr Azibert, one of the most senior judges at the court of appeal, was called in for questioning on Monday. Another judge, Patrick Sassoust, was also questioned, as was Mr Sarkozy's lawyer Thierry Herzog. (+ video) » | Tuesday, July 01, 2014
