Ghannouchi, who was arrested on Monday, was ordered detained following an investigation by the judge that lasted eight hours, lawyer Monia Bouali added. No date has been set for any trial or the next hearing.
Ennahda condemned the "unjust decision" and said it was intended to cover up failure in a stifling economic and financial crisis that is leading the country towards bankruptcy.
It said that targeting a "national symbol" (Ghannouchi) who has spent decades resisting dictatorship would not resolve Tunisia's problems and would not weaken the opposition.
Ghannouchi's official Facebook page published a comment by him after the judge's decision which said: "I am optimistic about the future ... Tunisia is free."
His lawyer said the decision to hold Ghannouchi in custody was pre-planned because of his expressions of opinion.
Since Tunisia's 2011 revolution, Ghannouchi has been a major political player, leading his party to participate in successive coalition governments with secular parties.
He has faced repeated rounds of judicial questioning over the past year on charges relating to Ennahda's finances and to allegations it helped Islamists travel to Syria for jihad, charges he and the party deny.