The supporters were detained before and during an attempt to protest outside parliament in favour of reform, the Brotherhood said on Monday.
Police took 84 prominent members from their homes on Sunday morning and picked up more than 150 demonstraters from the streets later in the day, Brotherhood leader Muhammad Mahdi Akif said.
Founded in 1928, the Brotherhood is one of the most influential Islamic political movements in the Arab world and probably the largest single opposition group in Egypt.
Thirty-four of the demonstrators have since been released, the Egyptian Organisation for Human Rights said later on Monday. A Brotherhood official confirmed that figure.
An Interior Ministry spokesman said police had detained 50 people in connection with the protests.
Thousands of police armed with sticks and shields were deployed in central Cairo from early Sunday and prevented most of the Brotherhood supporters from reaching parliament.
"The security apparatus is afraid of the power of the Brotherhood. They own the political street"
Member of Egyptian Parliament
But up to several hundred protested at other sites in the Cairo area, calling for an end to emergency laws and political and constitutional reforms.
In recent months, the authorities have allowed small public demonstrations by the Enough Movement, which wants to prevent President Husni Mubarak from seeking a fifth six-year term or arranging a transfer of power to his son Jamal.
Akef said the Muslim Brotherhood, which was formally banned in the 1950s but continues to operate, had a right to express its opinion on constitutional change and political reform because of its long history and wide support across the country.
Freedom of assembly
"So we were surprised on Sunday ... that the centre of Cairo was turned into a military barracks and a security arsenal, with tens of thousands of heavily armed troops," Akef added.
The Egyptian Organisation for Human Rights called on authorities to release the detainees immediately and said the police handling of the demonstration was a "danger signal" for the right to peaceful assembly and freedom of expression.
Mubarak (R) is thought to want
his son Gamal to succeed him
Analysts said the authorities took a tougher line against the Brotherhood because it is such a large organisation, probably with hundreds of thousands of supporters.
"The security apparatus is afraid of the power of the Brotherhood. They own the political street," Muhammad Alwan, a member of parliament from the liberal Wafd Party, said.
"The way to resist them is not to suppress them but by spreading freedom. Suppression leads to explosion."