Many hundreds of riot police in black on Wednesday surrounded the Judges Club in central Cairo, where a small group of activists had gathered to show support for judges demanding judicial independence and for two judges facing dismissal.
Plainclothes police then moved in and pulled the demonstrators out of the club one by one, witnesses said.
Women screamed and, as protesters shouted "Down with (President Hosni) Mubarak", the police pulled down the banners and an Egyptian flag the demonstrators had put on the club wall.
The Judges Club and the government have been heading towards confrontation since last year, when prominent judges revived their demand for a new law giving them financial and administrative independence from the Ministry of Justice.
The two sides also disagreed over the conduct of last year's presidential and parliamentary elections, which were meant to take place under full judicial supervision. Judges reported many abuses and in some cases outright fraud.
Two judges, Mahmoud Mekky and Hesham Bastawisi, are due to appear before a disciplinary committee on Thursday, accused of damaging the reputation of the judiciary in remarks about the elections on satellite television stations.
The Judges Club, an informal and independent institution, has called a general assembly on Thursday to support them.
The activists detained on Wednesday included Kamal Khalil, a leftist prominent in last year's demonstrations against Mubarak by the Kefaya (Enough) Movement, and journalist Ibrahim Sahary, another prominent Kefaya member.
The activists held on Wednesday
included two Kefaya members
The liberal Ghad Party of imprisoned opposition leader Ayman Nour said police also detained some of its members.
The opposition Muslim Brotherhood said police detained six of its members in the Mediterranean city of Alexandria on Wednesday because they helped organise a rally of 3,000 people in the city in solidarity with the judges.
About 3,000 riot police surrounded that meeting, witnesses said.