Fillon, who has slid from frontrunner to third in the race following "fake jobs" allegations, told French TV late Thursday that President Francois Hollande had headed a "secret cell" that was responsible for leaks against him.
It was a "scandal involving the state", said the 63-year-old former prime minister.
"The press has been flinging mud at me for two months now," Fillon told France 2, speaking as the first round of the two-stage election looms on April 23.
Fillon said that, according to a book out this week by "journalists who are far from being my friends", Hollande had obtained the contents of wiretaps linked to judicial investigations "which is totally illegal".
"We were looking for a secret cell and we found it," Fillon said, referring to the alleged source behind a slew of accusations of wrongdoing that have mainly been made in the Canard Enchaine, an investigative and satirical newspaper.
Hollande responded furiously to Fillon's accusations, saying he had exceeded the bounds of "dignity and responsibility" with his claims.
"I don't want to enter the electoral debate... but there is a dignity, a responsibility to respect," the president told French radio.
"Fillon is beyond that now."
French prosecutors have charged Fillon with several offences over accusations first made in the Canard Enchaine that he paid his wife Penelope 680,000 euros ($725,000) over 15 years for a fake job as his parliamentary aide.
It was revealed this week that the investigation has widened to include claims that the Fillons faked documents to support their case that she in fact performed duties to earn her salary.
- Embarrassing -
The claims have proved deeply damaging for Fillon, who clinched the rightwing nomination by campaigning as a "Mr Clean" untainted by the scandals that have beset his conservative rivals such as former president Nicolas Sarkozy.
After riding high in the polls at the start of the year, Fillon is now trailing far behind far-right leader Marine Le Pen and centrist candidate Emmanuel Macron, who are running neck-and-neck.
According to current projections, Fillon would therefore be eliminated in the first round on April 23.
Macron, 39, would rout 48-year-old Le Pen in the decisive run-off on May 7 if the election were held now, the same projections show.
One of the authors of the book in question, Didier Hassoux, disputed Fillon's claim, saying he was a candidate on the ropes who was making untenable claims.
"The only person who believes there is a secret cell at the Elysee is Francois Fillon," Hassoux said, using the name of the French president's official residence.
Immediately after Fillon's interview was broadcast, Hollande's office released a statement saying: "The executive has never intervened in any judicial process."
The justice ministry also hit back at the allegations, saying that "this presidential candidate has systematically voted against every proposal to protect the independence of the judiciary and the transparency of political life".
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)