His lawyers said they would appeal against the decision to formally investigate Sarkozy over allegations he took advantage of elderly L'Oreal heiress Liliane Bettencourt when she was weakened by poor health.
He was unexpectedly summoned on Thursday to the Bordeaux offices of Jean-Michel Gentil, the judge in charge of the case, for face-to-face encounters with at least four former members of Bettencourt's staff.
The surprise confrontation came over claims he had accepted envelopes stuffed with cash from the world's richest woman to fund his 2007 election campaign.
Gentil was seeking to establish how many times Sarkozy had visited Bettencourt during his successful campaign.
Sarkozy, 58, has always maintained that he visited Bettencourt's residence only once during the campaign, to meet her late husband. Members of the multi-billionaire's staff have, however, contradicted his version of events.
Sarkozy's lawyer Thierry Herzog lambasted the decision to pursue his client as "legally incoherent and unfair".
He said he would immediately initiate proceedings to have the charges dropped.
Back in November, Gentil and two other examining magistrates spent 12 hours questioning Sarkozy.
They decided not to formally charge him then but to continue investigating the allegations against him.
France's legal system has nothing that exactly matches the charges or indictments brought in English, or US courts; but an investigating judge's decision to place someone under judicial investigation is the closest equivalent.
Bettencourt is now 90. Medical experts say her mental capacity began to deteriorate from the autumn of 2006.
The allegation is that Sarkozy obtained significant amounts of money from her, breaching electoral spending limits and taking advantage of a person weakened by ill health.
Bettencourt's former accountant, Claire Thibout, told police in 2010 she had handed envelopes filled with cash to Bettencourt's right-hand man, Patrice de Maistre, on the understanding it was to be passed on to Sarkozy's campaign treasurer, Eric Woerth.
Woerth has already been charged in the affair.
Investigators suspect up to 4.0 million euros ($A4.98 million) of Bettencourt's cash subsequently made its way into the coffers of Sarkozy's UMP party.
Sarkozy lost immunity from prosecution when he was defeated in the 2012 presidential election by Socialist Francois Hollande.
French judges have already successfully pursued Sarkozy's predecessor as president, Jacques Chirac, who was convicted in 2011 on corruption charges related to his time as mayor of Paris.
Chirac, excused from attending his trial because of ill health, received a two-year suspended prison term.