Protesters in Cairo's Tahrir Square launched a sit-in on Friday following a rally to reclaim the revolution amid anger over the military rulers' handling of the transition.
Thousands had flocked to the epicentre of protests that toppled president Hosni Mubarak in February to demand an end to military trials of civilians, cleansing institutions of former regime remnants, amendment of a recently published electoral law and social justice.
Speakers at the main podium declared they would remain in the square until their demands were met, but some groups including the April 6 pro-democracy movement that had joined the protests refused to take part in the sit-in.
Meanwhile, around 300 people left Tahrir and made their way to the defence ministry in the Abbassiya neighbourhood. The army deployed troops to block them from reaching the building, a security official said.
A few dozen protesters then took the metro train to get closer to the defence ministry but were blocked at the entrance by military police.
Some demonstrators began to throw rocks and stones at the metro station, damaging the entrance, a security official told AFP, adding that "some arrests had been made" though the exact number was not immediately clear.
The powerful Muslim Brotherhood had said Wednesday that it would not participate in the demonstration.
Among the crowd on Friday was US film star Sean Penn, who was carrying an Egyptian flag accompanied by Egyptian movie star and political activist Khaled al-Nabawi.
Preacher Mazhar Shaheen, delivering the Friday Muslim prayer sermon, vowed to protect the goals of the revolution that toppled Mubarak.
He urged Egypt's military rulers to activate a law that prevents members of Mubarak's now dissolved National Democratic Party from running for public office.
Shaheen also called for the amendment of a new electoral law, which stipulates that two-thirds of parliament be elected on a list system and one-third as independents.
The aim would be "to prevent powerful individuals from controlling votes by hiring thugs to bully voters." Under Mubarak, influential businessmen associated with the regime were known to hire people to bribe or harass voters into picking them.
On Tuesday, the Supreme Council of the Armed Forces (SCAF), which took power when Mubarak was ousted, laid out the timetable for the first post-Mubarak elections, which will start on November 28 and take place over four months.
Presidential elections are expected to be held next year.
The Democratic Coalition, which groups dozens of parties including the powerful Muslim Brotherhood and the liberal Wafd party, have threatened to boycott the vote.
They fear the electoral laws will help old regime figures to return to parliament.
Under Mubarak, candidates affiliated with his party used patronage or pressure to garner votes.
Activists say that a proportional list system would help avoid that because voters would be electing candidates based on a party's political platform, circumventing candidates' personal power and influence.
On Thursday, six presidential hopefuls, including former Arab League chief Amr Mussa, issued a statement denouncing what they say is the military's extension of the transition period.
They called on SCAF to provide a clear road map, which would ensure that presidential elections are scheduled for no later than March 2012.
On the eve of Friday's protest, SCAF warned against any threat to democracy and national security and "those who seek to impede the democratic transformation that began with the call for parliamentary elections."
"Those who have called for the Friday (protest) bear the responsibility to organise and secure and protect all private and public property," SCAF said.
"Any encroachment on army units or camps or important establishments will be considered a threat to Egyptian national security and will be dealt with with the utmost firmness."
Military ruler Field Marshal Hussein Tantawi has repeatedly stressed the army's commitment to democracy, but protesters have maintained pressure on the military council he presides over because of the slow pace of change.
Thousands of protesters have gathered in Tahrir Square on an almost weekly basis to rally for speedy reforms.