The state prosecutor ordered the detention on Thursday of members of opposition group Gama'a al Islamiya - which was outlawed after leading an armed struggle against the government in the 1990s.
Security forces arrested the 11 men on Wednesday while they were meeting in a flat belonging to one of the group in the Giza district near Cairo.
The state security prosecutor accused them of having pamphlets supporting the ideas of Gama'a al-Islamiya (Islamic Group) and belonging to an illegal extremist organisation.
The prosecutor's order will keep the men in detention for an initial 15 days, although that period can be extended.
The arrests follow the detention of another group of 12 suspected Gama'a members in Cairo on Tuesday.
1997 - The Gama'a al Islamiya renounces violence
1997 - Gama'a al Islamiya renegade members kill 58 tourists in Luxor
2001-3 - Hundreds of Islamists arrested in state crackdown
Egypt, a key US ally, has arrested hundreds of people it accuses of Islamist activities since the September 11 attacks on the United States.
However, President Hosni Mubarak denies charges by human rights groups that he has used the US-led "war on terror" to stifle political dissent.
Gama'a al-Islamiya renounced violence in 1997, ending a five-year armed struggle for an Islamic state.
But the decision divided the group, and later that year some renegade members killed 58 foreign tourists in the southern town of Luxor.
Since then there have been no major attacks against foreign or government interests in Egypt.