Nicolas Sarkozy became the first former French president to be taken into formal custody as he was detained yesterday in an influence-peddling probe.
He was held for questioning into suspicions that a network of informers kept him abreast of a separate inquiry into alleged irregularities in his 2007 election campaign, a legal source said.
The conservative politician denies wrongdoing in a string of investigations which could derail his hopes of a comeback after his 2012 presidential election defeat by Francois Hollande.
Sarkozy arrived early yesterday to be quizzed by investigators at their offices in Nanterre, west of Paris, after his lawyer was held for questioning on Monday.
French government spokesman Stephane Le Foll said Sarkozy was "subject to justice just like everyone else".
Sarkozy is the focus of an investigation launched in February into whether he sought to use his influence to get information about a separate inquiry into allegations that late Libyan leader Muammar Gaddafi funded his 2007 election campaign. Investigators suspect Sarkozy and his lawyer kept tabs on the case by using a network of well-connected informants, which only came to light following phone taps.
Sarkozy has likened the magistrates behind the phone-tapping to the "Stasi" police of former Communist East Germany.
Two legal officers with prosecutor roles, Gilbert Azibert and Patrick Sassoust, are also being held for questioning. Investigators suspect Sarkozy had sought to get Azibert a promotion in exchange for information.
Allies from Sarkozy's conservative UMP party - in crisis because of questions over spending during the 2012 presidential campaign - jumped to his defence. "They have never imposed such treatment on a former president, with such a surge of hate," lawmaker Christian Estrosi tweeted.
Additional reporting by Associated Press