The arrests came after the country's largest opposition movement launched a series of protests demanding reform.
Police arrested 50 Brotherhood members in the northern Delta province of Munufiya early on Monday, after protests by the group the day before, officials in the prosecutors office said.
The Munufiya protesters were demanding more freedom and the release of detainees.
South of Cairo, another 33 members were detained in the city of Assiut, according to the Brotherhood.
Security officials said several dozen were arrested, but would not give the exact number.
The arrests sparked a protest by about 1000 Brotherhood supporters in Assiut, but the protest ended without incident, security officials said. The Brotherhood said 3000 people participated in the protest.
The government crackdown was launched after the Brotherhood began a series of large, public protests, joining other opposition groups in demanding political reforms in the government of President Hosni Mubarak.
The Muslim Brotherhood leader
is demanding urgent reform
For years, the Brotherhood has not held protests, seeking to avoid provoking the authorities.
But it held nationwide protests on Wednesday and Friday, sparking new arrests.
The group said 2000 supporters were detained, although police put the number at 750, more than 600 of whom were held without charge for further investigations.
One protester died on Friday in a stampede after police used tear gas to disperse protesters who allegedly threw stones at police outside the northern city of Mansura.
Four Brotherhood leaders were among those arrested, including Essam al-Erian, who was charged on Saturday with belonging to the leadership of a banned organisation, calling for protests and inciting sedition.
The Brotherhood, which calls for implementing Islamic law in Egypt, has been banned since the 1950s. It renounced violence in the 1970s and its operations have sometimes been tolerated.
Hosni Mubarak has been Egypt's
president since 1981
Meanwhile, a human rights report issued on Monday denounced widespread detentions by Egyptian authorities, saying police regularly torture and abuse prisoners and keep them in inhumane conditions.
The annual report by the Egyptian Human Rights Association for the Assistance of Prisoners said "2004 witnessed the detention of thousands of citizens".
More than 3000 were detained following 7 October bombings at the Sinai resorts of Taba and Ras Shitan, which killed 34 people.
The government has never confirmed the number of people detained since the Sinai bombings, the first significant attacks in Egypt since the 1997 massacre of tourists at Luxor.
Conditions in short-term and long-term detention facilities were characterised by "intentional brutality", the report said, adding that torture and abuse continued to be among interrogation methods adopted in police stations.
The report noted the arrest of 58 Muslim Brotherhood members in 2004 - including "the death of one of the detainees, engineer Akram al-Zohairi, and the torture of many of the detainees".
Egypt denies the existence of abuse in its prisons and says the arrests are necessary to maintain social stability.