A leader of one of the youth movements behind Egypt's 2011 uprising has been detained by security forces, officials have told reporters.
Ahmed Maher, co-founder of the pivotal April 6 movement, was held at Cairo airport "based on the orders of the general security department at the interior ministry", a security official told the AFP news agency.
He had just returned from Washington, DC, where he had addressed a New America Foundation conference entitled "Egypt in transition".
His passport was confiscated before he was taken for questioning, according to the official MENA news agency.
Maher stands accused of encouraging Egyptians to protest outside the home of Interior Minister Mohammed Ibrahim, a ministry official told MENA.
Four of the movement's members were arrested after the March demonstration, said spokesperson Amal Sharaf.
"I think they are punishing us and trying to stop us from what we are doing but of course they will not succeed because we will continue," she said.
I think they are punishing us and trying to stop us from what we are doing, but of course they will not succeed - because we will continue.
Hailed as heroes in the aftermath of the overthrow of former president Hosni Mubarak, members of the youth-led April 6 group have since reportedly split into two factions.
Analysts say that Maher's detention may provoke a reaction among youth groups, as tensions continue between activists and authorities.
April 6 had supported President Mohamed Morsi during the June presidential elections, but it has since become increasingly vocal in its opposition to the country's leader - who they accuse of pursuing Mubarak's policies and
betraying the revolution that brought him to power.
Two years after Mubarak's ousting, 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.
A myriad of charges and complaints have been levied in recent months against activists, journalists and TV personalities, including well-known satirist Bassem Youssef, for insulting President Morsi.
Earlier this month, authorities arrested Ahmed Douma, a leading activist, and referred him to court for allegedly insulting the president in a TV interview.
The United States sends $1.3 billion in military aid to Egypt each year. Washington officials in March expressed concern that arrest warrants had been issued for political activists.