The 75 newly arrested Muslim Brotherhood supporters were detained late on Wednesday and early on Thursday, and prosecutors ordered them held for 15 days for investigation.
A total of 754 members of the outlawed organisation are in detention over protests the group held in May in several Egyptian provinces, part of a growing opposition campaign for reform.
Detention was extended for 15 days for four leading figures arrested on 6 May, including Essam el-Eriyan, one of the most prominent young leaders of the group.
On Wednesday, the banned but tolerated Muslim Brotherhood said 753 of its members had been arrested by the police in raids across the country.
The raids targeting the Muslim Brotherhood, which has been behind a series of pro-reform protests against President Hosni Mubarak, were conducted in nine governorates.
"The number of members of the Brotherhood held ... now totals 753 people, including 28 people arrested on Saturday and some 60 arrested on Tuesday," the group said in a statement.
Protests against President
Mubarak have been growing
The crackdown came a day after the group and three main opposition parties announced they would boycott a disputed referendum on a constitutional amendment to the mechanism of electing Egypt's next president.
The chief of the Muslim Brotherhood issued a statement saying Egypt's security apparatus has launched a campaign against the group's members.
Speaking to Aljazeera, Muslim Brotherhood chief Mohammed Mahdi Akif condemned the arrests and vowed to press ahead with the "struggle for the nation's welfare".
"All Egyptian people reject this regime and this rude, obstinate and tyrannical style," he added.
Earlier, judicial sources said 56 Muslim Brotherhood members had been seized on Tuesday, accused of belonging to a banned group and "attempting to mobilise students and workers" against Mubarak's 24-year-old government.
Egypt has witnessed a wave of protests, many of them organised by the Muslim Brotherhood, over Mubarak's failure to engage in true democratic reforms.
"All Egyptian people reject this regime and this rude, obstinate and tyrannical style"
Mohammed Mahdi Akif,
Muslim Brotherhood chief
Several members were hurt by police during rallies and one died after inhaling tear gas, according to rights groups and the Muslim Brotherhood.
Egypt's opposition groups are also demanding the lifting of emergency laws imposed after the assassination of Mubarak's predecessor Anwar al-Sadat 24 years ago.
They have also called for a boycott of a 25 May referendum on a constitutional amendment allowing for the first competitive presidential polls, saying the rules are still too restrictive.