Brotherhood members running as independents to circumvent a ban on the group won nearly a fifth of the 454-seat lower house of parliament in 2005.
Analysts say the government fears that unless it stops the Brotherhood now, the group will make more electoral gains that could help it eventually mount a serious challenge to Hosni Mubarak's ruling National Democratic Party.
"It is all because of the constitutional amendments," a Brotherhood spokesman said about the fresh arrests.
The constitutional amendments would weaken the role of judges in monitoring elections. An anti-terrorism clause would give police sweeping powers of arrest and broad authority to monitor private communications.
The proposed laws would also ban political activity based on religion, quashing Brotherhood hopes of acquiring legal standing as a recognised political party.
The Brotherhood believes the proposed amendments, which parliament is expected to pass shortly, aim to block the group from mainstream national politics. The Brotherhood operates openly in Egypt despite being officially banned.
Thursday's arrests followed sweeps earlier in the week that netted 18 Brotherhood members including Mahmoud Ghozlan, a senior leader and a member of the group's executive Guidance Council.
More than 300 Brotherhood members are now in detention, including third-in-command Khairat el-Shatir, who was referred last month to a military trial along with 39 others on charges including money laundering and terrorism.