Taunting rows of riot police, demonstrators called on Egyptian President Husni Mubarak to release Ayman Nur, the leader of the opposition Hizb al-Ghad (Party of Tomorrow).
"Mubarak is a disgrace and a traitor!" the crowd chanted outside the Egyptian Journalists Syndicate building on Wednesday, as more than 100 riot police looked on.
Nur, the sole representative of al-Ghad in parliament, was arrested last week after a court ordered he be detained for 45 days on accusations of forging nearly 2000 signatures to secure a licence for his political party.
He strongly denies the forgery accusations.
His arrest drew criticism from Washington last week. The State Department said it was "incongruous" for the Egyptian government to arrest an opposition party leader on the eve of a national dialogue between the government and its political rivals.
"His aim is to change the system of government, so a president can only serve for two terms, and no more"
Al-Ghad party spokesman
Al-Ghad party spokesman Walid Riyad said Nur was arrested because of he was challenging Egypt's political hierarchy.
"His aim is to change the system of government, so a president can only serve for two terms, and no more," Riyad said.
Nur is among those government opponents who have called for the constitution to be amended to allow more than one candidate to run for the presidency.
Egypt holds presidential referendums in which people vote yes or no for a single candidate, who has been approved by parliament.
Mubarak, Egypt's president since 1981, has hinted that he would stand for a fifth term in office later this year. There have been some protests against his re-election.
Mubarak's ruling National Democratic Party met on Monday with opposition parties for talks on political reform in Egypt.
Academics and opposition newspapers have accused the government of rigging elections. The parliament has been dominated by the ruling party since political parties were revived in the 1970s.