Egyptian security forces, backed by men in plain clothes armed with batons, charged a group of around 200 demonstrators protesting on Saturday against President Hosni Mubarak's intention to seek a fifth six-year term.
Several demonstrators were badly beaten and close to 40 detained.
Senior leaders of the opposition Kefaya (Enough) movement were released after a few hours but 21 remained in custody, prompting the remaining protesters to start a sit-in in front of the journalists' union that continued on Sunday morning.
"Twenty-one are still being held at a military camp in Al-Darasa in Cairo. We will continue the sit-in until they are released," opposition lawyer Amir Salem said.
The Interior Ministry issued a statement saying demonstrators had ignored warnings to halt their protest and had thrown stones at security forces.
Protesters denied there was any stone-throwing, and reporters on the ground saw no such incident.
Some analysts say the heavy-handed techniques show the government will play tough in the coming elections and is not bothered by US criticism.
"The regime is fighting for its survival and will do anything to prevail," Egyptian-American sociologist Saad Eddin Ibrahim said of Saturday's clashes.
US State Department spokeswoman Nancy Beck said she had no comment on Saturday's incidnet, but Washington criticised the government after its supporters, with police standing by, attacked reform activists during a referendum in May.
A protestor lies on the ground
after clashing with riot police
US Secretary of State Condoleezza Rice said the government must "make certain that people can associate and can peacefully petition".
Nabil Abd al-Fattah, an analyst with the Al Ahram Centre for Strategic and Political Studies in Cairo, said: "The government feels American pressure is easing up in return for its supporting the American policy in the region and bolstering (US) legitimacy in Iraq."
He said the government is no longer worried about television channels broadcasting pictures of pro-government men beating protesters in the street, as happened after the May referendum.
"The upcoming summit in Sharm al-Shaikh is to legitimise the American policy in Iraq and Israeli policy in the Palestinian territories," Abd al-Fattah said, referring to the Arab Summit that Mubarak has called to discuss terrorism, Iraq and the Palestinian issue.
"We condemn the terrorism which killed about 100 people in Sharm, but we equally condemn the regime's terrorism, which has killed the dignity and spirit of millions of Egyptians," Kefaya spokesman Abd al-Halim Qandil said.
"This regime, which keeps accusing the opposition of being foreign agents, is America's No 1 agent," Qandil added, referring to the government's support of US policies in the Middle East.
In his speech on Thursday, Mubarak said he would introduce reforms to enhance Egyptians' rights as "free citizens in a democratic country".
Saad Eddin Ibrahim said such comments were "window dressing".
"In every test, the police nature of the regime is exposed, defying the nice empty words and pledges it makes," Ibrahim said.