Egypt's security forces have arrested at least 30 activists from the Kefaya opposition movement who were demonstrating against proposed constitutional amendments.
Several were released from custody on Friday night, but opposition party sources told Al Jazeera up to 20 were being detained until court hearings are held on Saturday.
The arrests come a day after similar security actions against the banned-but-tolerated Muslim Brotherhood organisation.
At least 200 people eventually gathered outside the headquarters of the opposition Tagammu party near Talaat Harb Square after state security forces had forced protesters out of the main streets.
The protesters chanted pro-reform slogans and accused the government of pushing through a referendum on constitutional amendments which they said will ultimately strengthen the ruling National Democratic party's hold on power.
Kefaya, Tagammu and the al-Ghad, party of imprisoned opposition leader Ayman Nour, had called for the rally weeks earlier.
Rows of state Central Security Forces cordoned off the area and prohibited access to the adjacent Mahmoud Bassiouny Street.
They began arresting demonstrators soon after they assembled on a roundabout in Tahrir Square, and by mid-evening, a dozen activists were removed from the area as they began to address the small crowd.
"We're staying for as long as it takes to free our imprisoned brothers"
"They took them from the heart of the protest," Sayid Abdel Fattah, 63, a lawyer and Kefaya activist, said of the massive state security presence.
"Now we don't have a protest in Tahrir. The thugs scared everyone away."
At least one Kefaya activist was beaten in the crowd by plainclothes officers and dragged across the street. He could be heard screaming from inside a state security truck before it drove away.
Later, Kefaya leaders insisted on addressing the demonstration outside the opposition party headquarters.
Abdel Aziz el-Husseiny, media co-ordinator of Kefaya, told the crowd: "The government fears the word of truth from the activists and [the possibility] that Kefaya's voice could ready the Egyptian people for change.
"The police presence is always this size. We find this huge number even if it is an opposition official in a building with 20 attendees."
Israeli, American flags burned
When the crowds formed outside the Tagammu party, some burned American and Israeli flags, setting off clashes with state security.
"There's no freedom of expression, people are getting arrested all of the time," said Noha Ahmed, a Kefaya activist, as she was being pushed by police out of Tahrir Square.
When demonstrators heard that several of their members had been detained they staged a sit in outside the Tagammu party headquarters demanding their release.
"We're staying for as long as it takes to free our imprisoned brothers," said Mohamed Sharqawi during the sit-in. Sharqawi, a blogger and activist, claimed in 2006 that he was detained and sexually abused by state security last May during protests calling for an independent judiciary.
The protesters began to disperse early on Friday morning when police officials said they had allowed the detainees the use of their mobile phones.
However, senior police officials at the scene refused to comment on the arrests.