Thirteen senior members of the Islamist opposition movement in Egypt, the Muslim Brotherhood, have been detained.
They include its deputy leader, Mahmoud Ezzat, the organisation says.
The Brotherhood is officially outlawed in Egypt but its members have many seats in parliament, sitting as independents.
A spokesman for the group said the arrests were an attempt by the authorities to thwart its preparations for elections later in the year.
Supporters are routinely arrested.
The Brotherhood has been banned from open political activity since 1954, and leading activists are frequently arrested and imprisoned by the authorities.
Despite this, Brotherhood members standing as independent candidates won 20% of the seats in the last parliamentary election in 2005, its best ever result.
In January, the movement named Mohammed Badie, a 66-year-old veterinary professor, as its new leader.
He has said that the Muslim Brotherhood was not an adversary of the Egyptian government and that he would try to avoid confrontation with it.
The Muslim Brotherhood has influenced Islamist movements around the world with its model of political activism combined with charity work.