The opposition Islamist group in Egypt, the Muslim Brotherhood, says at least 73 of its members have been arrested in overnight raids across the country.
The Muslim Brotherhood is the leading opposition group in Egypt
Security sources said those held were detained for belonging to an illegal organisation and for possessing anti-government literature.
The Muslim Brotherhood is outlawed in Egypt but is normally tolerated.
The group says the arrests are a pre-emptive move ahead of April's vote for the upper house of parliament.
"This is an attempt to marginalise the role of the Brotherhood in Egyptian political life, an attempt to impede our political path," Mohamed Habib, the Brotherhood's deputy leader, told the Reuters news agency.
The raids came days after one of the Brotherhood's most senior figures, Khayrat al-Shatir, was ordered to stand trial in a military court on charges of money-laundering and financing an illegal group.
Thirty-nine other people are also accused.
Also on Thursday, Human Rights Watch, a US-based organisation, called on the Egyptian authorities to free hundreds of Brotherhood members who it said were detained "solely for exercising their rights to freedom of expression and association".
The Muslim Brotherhood ran in the legislative elections in November and December 2005, with candidates standing as independents, and won 88 of the 454 seats in parliament.