Former French President Nicolas Sarkozy has denied any wrongdoing and says he has not "betrayed" his country after being put under formal investigation for corruption.
Mr Sarkozy told Europe 1 radio the country's legal system was "being used for political means".
"The situation is sufficiently serious to tell the French people where we stand on the political exploitation of part of the legal system today," he said.
He added: "I say to all those who are listening or watching that I have never betrayed them and have never committed an act against the Republic's principles and the rule of law."
One of the allies of Nicolas Sarkozy has expressed doubts over the fairness and impartiality of one of the magistrates investigating the ex-French president over allegations of corruption.
"I question the impartiality of one of the judges," Christian Estrosi, the mayor of the southern city of Nice told France Info radio, accusing Hollande's government of having whipped up "an atmosphere of hate".
Former French President Nicholas Sarkozy has been placed under formal investigation by investigators after a hearing late on Tuesday night.
.As ITV News Europe Editor James Mates reports, this is one level below an official charge.
Fmr Pres #Sarkozy placed under formal investigation early today for influence peddling, corruption. 1 level below being officially charged.
Under French law, when a suspect is placed under formal investigation, he or she is then examined by a judge, who determines whether there is sufficient evidence for the suspect to be charged.
Former French President Nicolas Sarkozy has been escorted by police to a hearing before a judge in relation to allegations of corruption, influence-peddling and the "breach of professional secrets".
Mr Sarkozy has not been charged but he is now under "formal investigation", as are his lawyer and a judge suspected of being connected to the allegations.
Former French President Nicolas Sarkozy is under formal investigation relating to allegations of corruption.
He is suspected of influence peddling, corruption and benefiting from "the breach of professional secrets," Reuters reports the prosecutor's office as saying.
Mr Sarkozy was earlier held for questions over suspicions he received leaked details of an inquiry into alleged irregularities in his 2007 election campaign
He and his lawyer appeared before a judge on Tuesday evening before being released in the early hours of the morning.
The investigation may be a major blow to his hopes of returning to front-line French politics following his defeat to Francois Hollande in the 2012 presidential election.
Former French President Nicolas Sarkozy is being held for questioning over suspicions he received leaked details of an inquiry into alleged irregularities in his 2007 election campaign.
It is the first time a former head of state had been held for questioning in modern French history and is the latest blow to Sarkozy's hopes of a come-back after his 2012 election defeat by Francois Hollande.
The conservative politician denies all wrong-doing in a string of investigations involving him.
Asked about the matter, government spokesman Stephane Le Foll said Sarkozy was "subject to justice like everyone else."
Former French president Nicolas Sarkozy has been detained for questioning, as part of an investigation into corruption.
Europe Editor James Mates has tweeted:
Former French President Nicolas Sarkozy is being held for questioning, as part of a corruption investigation, Reuters reports.
Sarkozy arrived early this morning to be quizzed by investigators at their offices in Nanterre, west of Paris, after his lawyer was held for questioning on Monday.
"Mr. Sarkozy has been summoned to Nanterre and is being held for questioning," a source told Reuters.
French President Francois Hollande is sitting next to his predecessor Nicolas Sarkozy at the memorial service for Nelson Mandela.