Former French President Nicolas Sarkozy says France's legal system is being used for political means after he was accused of corruption.

Mr. Sarkozy (59) was placed under formal investigation - a step which often but not always leads to trial - after he was questioned for 15 hours by police.

It marked the first time a French ex-head of state has been taken into custody in a criminal investigation and could wreck his hopes of a political comeback.

He was detained at a police station in Paris over a suspected attempt to illegally influence judicial proceedings in one of a string of corruption cases he is implicated in.

Later, the Conservative politician denied any wrongdoing in an interview with Europe 1 radio and claimed the allegations are politically motivated.

"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.

"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".

The allegations are a major setback to his viability as a candidate in France's 2017 elections following his 2012 defeat by Socialist rival, Francois Hollande.

Magistrates are looking at whether Mr. Sarkozy used his influence to secure leaked details of an inquiry into alleged irregularities in his victorious 2007 campaign.

He is suspected of corrupting officials and benefiting from breach of professional secrets.

Mr. Sarkozy's supporters have cast doubts over the impartiality of one of the investigating magistrates, with Nice mayor Christian Estrosi telling France Info state radio that Mr Hollande's government had whipped up "an atmosphere of hate".

Prime Minister Manuel Valls dismissed accusations of a plot.

Investigating magistrates have a unique and powerful role in French law, both gathering evidence and determining whether it is solid enough for a trial.

After the inquiry, the magistrates can drop the case for lack of proof or "charge" the accused, sending the case to trial. If convicted of the charges, Mr. Sarkozy could face a sentence of up to 10 years in prison.