The Brotherhood - which has been banned since 1954 - told reporters during a packed press conference on Sunday that it would keep staging protests to push for political reform from Hosni Mubarak's 24-year-old presidency.
"We are not getting into a confrontation, but we will hold demonstrations soon demanding change," Muhammad Mahdi Akef said.
"We are reaching out to oppressed Egyptians who reject the oppressive way in which they are being ruled," he added.
Earlier on Sunday, about 3000 female Brotherhood supporters also gathered in Egypt's second city to demand democratic reforms.
The marchers called for the lifting of emergency laws imposed 24 years ago after the assassination of president Anwar al-Sadat and increased political freedom.
Egypt's largest Islamist group says police have detained more than 2000 of its supporters during nationwide protests held on Wednesday and Friday.
Police said they had arrested only 750, including four leading members, and had ordered 617 detained for an additional 15 days without charge for further investigations.
The frequency of anti-government protests has increased since December, and the Brotherhood - after long avoiding confrontation with the government - has since March joined more secular reform movements demanding reform.