Egypt detains leading youth activist Ahmed Maher
Egyptian security forces have detained a prominent activist on suspicion of inciting an anti-government protest.
Ahmed Maher, who leads the April 6 Youth Movement, was arrested at Cairo airport after flying back from the US, officials said.
The April 6 Youth Movement was at the forefront of protests that overthrew former leader Hosni Mubarak in 2011.
Mr Mubarak's retrial on charges linked to the revolt is due to begin on Saturday.
He is charged alongside a former interior minister and six former security chiefs with complicity in the killing of hundreds of protesters.
Mr Mubarak will also face charges of financial corruption alongside his two sons, Alaa and Gamal.
The official Mena news agency said Mr Maher's passport was confiscated and he was ordered to be detained for four days.
A security official quoted by AP news agency said Mr Maher was accused of "incitement" following a demonstration in March against the country's interior minister.
Protesters hurled underwear at the minister's house following a police crackdown on the activist group.
Mr Maher is being held on the premises of the prosecutor's office in Cairo's eastern district of Nasr City, the official said.
Ingi Hamdi, a senior member of the April 6 Youth Movement, denounced the arrest and pledged a "tough" response
"This is part of a series of arrests targeting activists... to undermine their reputation," she said.
Correspondents say the youth movement - hailed as heroes in the aftermath of Hosni Mubarak's downfall - has since split into two factions.
It supported President Mohammed Morsi during the June presidential elections but later became increasingly vocal in its opposition to the Islamist leader.
Days before his arrest, Mr Maher had expressed regret for the group's alliance with Mr Morsi and his main backers, the Muslim Brotherhood.
He wrote on the group's website: "Now we are being treated as traitors and our image has been tarnished and we are sent to prisons by those we defended."