Former French president Nicolas Sarkozy is being 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.
Mr Sarkozy arrived early this morning to be questioned by investigators at their offices in Nanterre, west of Paris.
His lawyer Thierry Herzog was held for questioning yesterday.
The investigators are seeking to establish if Mr Sarkozy, with the help of Mr Herzog, attempted to pervert the course of justice.
Asked about the matter, French government spokesman Stephane Le Foll said Mr Sarkozy was "subject to justice just like everyone else".
Local media said it was the first time a former head of state had been held for questioning in modern French history.
The conservative politician denies wrongdoing in a string of investigations which could derail his hopes of a come-back after his 2012 presidential election defeat by Francois Hollande.
The former leader is the focus of an investigation, which began 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 Mr 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.
Mr 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 Mr Sarkozy had sought to get Mr Azibert a promotion to Monaco in exchange for information.
The case is one of six legal investigations involving Mr Sarkozy, including a new one this year into separate irregularities in his unsuccessful 2012 election campaign.
Supporters of Mr Sarkozy claim the investigation is politically motivated.
Sebastien Huyghe of Mr Sarkozy's centre-right UMP party said: "Every time Nicolas Sarkozy's political return was mentioned, it was followed by a judicial episode, as if someone wanted to prevent his comeback."
However, the government has dismissed this long-running accusation of judicial manipulation.
Julien Dray of President Hollande's socialist party said: "I think the best favour Nicolas Sarkozy's friends can do is to let justice take its course."