One of the top leaders of Egypt's opposition Islamist movement, the Muslim Brotherhood, has been detained.
The Muslim Brotherhood is the leading opposition group in Egypt
Police also rounded up about 10 other prominent members and dozens of students in dawn raids.
Khairat al-Shatir is one of two deputies to Brotherhood leader Muhammad Akef, and was taken from his home in north-eastern Cairo, the capital.
The group is officially banned, but its supporters make up parliament's largest opposition group and it is tolerated.
Mr Shatir is the most senior member of the group detained by the authorities since Secretary-General Mahmoud Ezzat was released last year after three months in jail without trial.
Officials have not said why the latest arrests were carried out, but correspondents said it might be related to recent newspaper reports suggesting the movement was setting up a military wing.
"The number of students arrested was not specified, but it could be as high as 180," a security official told the AFP news agency.
"At least three professors and several student leaders were taken during this sweep," a Muslim Brotherhood official said.
"The 180 students were detained before dawn during a sweep at the al-Safa campus, an annex of al-Azhar [Islamic university]."
Islamist students at the university organised a military-style march there on Sunday, dressed in black uniforms.
The Muslim Brotherhood ran in the legislative elections in November and December 2005, with candidates standing as independents, and won 88 of the 454 seats in parliament.
Essam al-Aryan and Muhammad Morsi of the Muslim Brotherhood's political bureau were released earlier this week, having been arrested six months ago.