"Members of the brotherhood were arrested Friday in Alexandria, some of them as they were eating in a restaurant," Issam al-Aryan, a spokesman for the group, said.
He said that among those rounded up was Helmi al-Gazzar, a local leader in Cairo who was also picked up during a wave of demonstrations last year and jailed for five months.
According to the movement's website, another local brotherhood official, Hassan al-Brens, was also detained in the latest sweep.
A statement posted on the site said: "These arrests are the result of an escalation by the Egyptian government, which is cracking down on the Muslim Brothers in response to the movement's activities in parliament."
Thirty Brotherhood members have been detained since the beginning of last month, including Rashad Mohammed el-Bayoumy, a 70-year-old university professor and senior member of the group.
The Islamist movement is officially banned in Egypt but it fielded candidates as independents in last year's parliamentary polls and won a fifth of seats in the People's Assembly.
With 88 of the chamber's 454 seats, the Muslim Brotherhood is by far the largest opposition force and has adopted a stance of challenging the government's policies during parliamentary sessions.
The Egyptian authorities can hold detainees for up to six months without trial under much-criticised emergency laws introduced in 1981 after Islamist militants assassinated Anwar Sadat, the president.
The Muslim Brotherhood's official site in English: http://www.ikhwanweb.com/Home.asp?zPage=Systems&System=PressR&Lang=E