Former President Nicolas Sarkozy has been hauled in for questioning, and France is suddenly in uncharted territory.
French presidential candidates Hollande and Sarkozy clash in television debate.
Nicolas Sarkozy and Francois Hollande go head-to-head in the only televised debate before Sunday's vote.
The French far-right leader Marine Le Pen has addressed a rally in Paris. She told her voters to make their own choice in the May 6th presidential election runoff. The two leading candidates - Nicolas Sarkozy and François Hollande - have been competing for the votes of her supporters.
Far Right leader Marine Le Pen has told supporters in France to vote as they wish. She said: "You are free citizens", not endorsing Sarkozy.
President Nicolas Sarkozy has made an impassioned plea for five more years at a final rally before the first round of voting.
As former French president Nicholas Sarcozy is released from detention by French authorities, he denies any wrongdoing and says the country's justice system has been 'politicised', reports Europe Editor James Mates
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.