Aljazeera's correspondent reports that 12 leaders of the group were detained in the Mediterranean coastal city of Alexandria, nine in Tanta, nine in Kfr al-Shaikh and one in Cairo.
Other reports cite Ali Abd Al-Fattah, a leading member of the group, as saying some of the detainees were prominent members and the rest from middle ranks.
He said police told the detainees they were arrested for belonging to an illegal group.
Police officials and the prosecutor's office could not immediately be reached.
The arrests apparently are part of a longstanding government crackdown on the brotherhood that, members say, stems from their continued protests against US involvement in Iraq and support of the Palestinians in their resistance against the Israeli occupation forces.
The Muslim Brotherhood was banned in 1954 for advocating an Islamic state for Egypt. Today, it says it supports peaceful means towards change and is generally tolerated by the state, with occasional crackdowns.
Brotherhood members hold seats in parliament, though they are not permitted to run under the group's name and are elected as independents.
Abd Al-Fattah identified some of those who had been arrested as publisher Gamal Saadi Madhi and businessman Midhat Ahmad al-Haddad.
They each had spent three years in prison between 1995 and 1999. Another detainee, Ibrahim Said Muhammad, an engineer, had been arrested several times. Al-Muhammadi Said Ahmad, a candidate in 2000 parliamentary elections, was also arrested.
Muhammad Usama, an engineer, and Gamal Nassar, a doctor, were arrested in Cairo, according to al-Fattah.
Sources told Aljazeera the arrests came in the wake of a new campaign against the brotherhood members which had also resulted in the closing down of their website.