By Christian Fraser
BBC News, Cairo
A German-Egyptian activist has been detained north of Cairo during a rally in support of Palestinians in Gaza.
Philip Rizk has been an outspoken activist on Palestinian issues and lived in Gaza for two years.
He previously worked on aid projects with Canon Andrew White, special envoy to the Middle East for the Archbishop of Canterbury.
Eyewitnesses said he was bundled into a white van with no licence plates, which then sped off.
The German embassy has been informed of his detention and is trying to locate him.
His family say they have been tipped off that he is now being held by Egyptian secret police at an undisclosed location.
Over the weekend a delegation of lawyers and activists filed charges with the public prosecutor's office, relating to his kidnapping by three state security officers.
Mr Rizk writes a popular blog, "Tabula Gaza", and has just completed a short documentary about non-violent protest against Israeli operations in the Gaza Strip.
It is not known why he would be of special interest, though in past weeks a number of bloggers and activists from the Muslim Brotherhood - who campaigned for the re-opening of the Rafah crossing between Egypt and Gaza - have been formally charged with "forming a criminal group belonging to Hamas".
One of those arrested was Gamal Abdel Salam, who heads the Egyptian Doctors' Syndicate Relief Committee, which organised a number of convoys to the Gaza Strip.