Egypt's security forces on Friday detained the founder of one of the key youth movements behind the 2011 uprising that toppled the former regime upon his arrival at Cairo Airport from Vienna, a security official told AFP.
Ahmed Maher of the April 6 movement was arrested at the airport "based on the orders of the general security department at the interior ministry," the official said.
His passport was confiscated and he was transferred to the prosecution for questioning, the official MENA news agency said.
A source at the prosecutor's office later said that Maher was ordered kept in detention for four days as part of the investigation, adding that he was accused of "incitement to protest outside the house of the Egyptian interior minister," Mohammed Ibrahim.
"The prosecution has decided to jail Ahmed Maher for four days as part of the investigation," the source told AFP.
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Ingi Hamdi, a member of the political bureau of the April 6 movement, denounced the arrest of Maher.
"This is part of a series of arrests targeting activists... to undermine their reputation," Hamdi said, pledging a "tough" response.
Earlier the movement's spokesman Mohammed Adel said he was able to contact Maher soon after his detention at the airport "but we have not been able to reach him since."
Hailed as heroes in the aftermath of the overthrow of former president Hosni Mubarak, the youth-led group has since splintered into two factions.
April 6 had supported President Mohamed Morsi during the June presidential elections, but since then it has become increasingly vocal in its opposition to the Islamist leader who they accuse of pursuing Mubarak's practices and betraying the revolution that brought him to power.
Two years after the uprising that overthrow Mubarak, Egypt is still trying to find its footing in a declining economy and often violent protests that have kept away much needed tourist revenues and foreign investments.