As legislative elections draw near -- the first polls since Mubarak was ousted in February -- protesters are demanding more control over the constitution the new parliament is set to draft.
They want the withdrawal of a government document that proposes supra constitutional principles, which could see the military's budget shielded from public scrutiny.
Friday's protests are led by the powerful Muslim Brotherhood and groups of varying political stripes under different banners who all agree that the military must transfer power to a civilian government as soon as possible.
"The people want a timetable for the handover of power," read one large banner hanging over the square.
Delivering the Muslim prayer sermon, imam Mazhar Shahin urged protesters to keep defending the goals of the revolution.
"Perhaps those who rule us think we will forget our cause with the passage of time. They are deluded and mistaken," he warned the ruling Supreme Council of the Armed Forces (SCAF), which took power when Mubarak was ousted.
"We reject the imposition of dictates on the people, we reject Silmi's document. No voice can drown out the voice of the people," Shahin told the crowd.
The contested government document, presented by Deputy Prime Minister Ali Silmi, drew fire from most quarters for including clauses that removed the military's budget from parliamentary oversight and allowed the SCAF a final say on military-related matters.
The government revised the draft, but Islamists, who organised a mass protest in July against such a charter, have rejected the very idea of a document that would limit parliament's authority to draft the constitution, branding the articles undemocratic.
The Brotherhood, through its Freedom and Justice Party, may emerge as the largest bloc in the election, the first since the fall of Mubarak.
"Those who fear Islamist movements in Egypt, I tell them don't be scared of Islam in Egypt," Shahin said.
"Egypt is Islamic, like it or not... We want a civic democratic state with an Islamic vision that allows people to practise their rights and democracy," he said.
Adham Hani left his home in the central province of Minya to join the Tahrir protest.
"We need the cancellation of the constitutional principles, we need a date for presidential elections," said the 25-year-old.
"Parliament will have no sovereignty as long as the SCAF is in charge," Hani said.
Close by, the head of the fundamentalist Gamaa Islamiya, Tarek al-Zomor, told AFP that the constitutional principles were a "circumvention of the people's will."
"We are here to stress the necessity of a timetable to civilian rule. If that doesn't happen, then it confirms the conspiracy to rob the revolution," Zomor added.
The SCAF, which took charge after Mubarak's ouster and suspended the constitution and parliament, says it will hand over power once a new president is elected.
Parliamentary elections will start on November 28 and are expected to end in March.
Friday's demonstration comes a day after 25 people were injured when a Coptic Christian march came under attack by assailants throwing stones and bottles.
Copts, who make up roughly 10 percent of Egypt's 80 million people, complain of discrimination in the Muslim-majority country.
There has been a spike in sectarian clashes since Mubarak was toppled.