Egyptian security forces arrested Muslim Brotherhood spokesman Ahmed Aref early Thursday.
"Security forces arrested Aref without any charge," the Brotherhood's Freedom and Justice Party (FJP) said in a statement.
The party added that Aref was detained because "coup leaders are afraid of any opposition".
A security source said that Aref was arrested in a flat in Nasr City in eastern Cairo, according to Egyptian television.
Aref's wife said that her husband was taken from his house by security forces before dawn.
"Six security vehicles and 20 troops raided the house and searched everything but found nothing except the iPad which he uses to publish comments on current developments," Aref's wife, Eman, said via Facebook.
Ali Khafaja, the FJP youth secretary in Giza, described Aref's detention as an "honor for Brotherhood youth."
Egyptian security forces have cracked down on Muslim Brotherhood leaders since last week's violent dispersal of two major protest camps set up by supporters of ousted President Mohamed Morsi.
Muslim Brotherhood Supreme Guide Mohamed Badie was arrested early Tuesday in a flat in Nasr City. FJP media adviser Mourad Ali was also arrested on Wednesday while trying to leave the country to Italy.
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