Sources close to the investigation said late yesterday that the men were accused of discussing a potential hostage- taking or machine gun attack from their cell at the Fresnes prison south of Paris.
One of the suspects is a 28-year-old from Cameroon described by authorities as an Islamic State group sympathiser, while the other is a 22-year-old Frenchman. Both were set to be released this month.
The Cameroonian was believed to have been in contact with a person in Iraq or Syria, where the Islamic State's self- proclaimed "caliphate" has lost swathes of territory.
Both were behind bars for non-terror offences and are suspected of being radicalised while serving their sentences. They were charged Friday with being part of a terrorist conspiracy.
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The Cameroonian, who was due to be released today, confirmed to investigators that he was planning on carrying out an attack, a source said.
The Frenchman was due to be released next week.
Terror investigators moved to arrest them after a 44- year-old man from the French island of Reunion in the Indian Ocean, who was in contact with the pair, indicated he was "planning on heading to the mainland to offer them logistical support".
"What was revealed yesterday is testament to the urgency of the situation," she told Europe 1 radio.
In August, a radicalised prisoner attacked two guards at the Osny prison north of the capital in what was seen as France's first jihadist assault within a jail.
The prisoner, who was behind bars for attempting to travel to jihadist-held territory in Syria, told investigators he "wanted to kill a prison guard" in the name of IS.
France has suffered a string of jihadist attacks over the past two years that have left more than 240 people dead.
Fresnes is one of three French prisons under special watch for radicalisation, where prisoners are assessed for signs of extremism before their arrival.
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