Hundreds of pro-democracy activists were set to spend the night in the square, as security forces maintained a discreet presence on side streets.
At the noon Muslim prayer, Sheikh Mazhar Shahin urged protesters to "keep working to achieve the honourable goals of the revolution which are 'change, freedom and social justice'."
Speaking from a mosque in Tahrir, he called for a "strong government able to fulfill the demands of the people who revolted for a dignified life."
Tents were pitched in the middle of the square, and a large sun shade provided some relief from the scorching sun and temperatures of 37 degrees Celsius (about 99 Fahrenheit).
"Our revolution continues," read one banner.
Holding a large sign, one man complained: "We haven't felt any change. We removed Mubarak and got a field marshal."
He was referring to Hussein Tantawi, head of the Supreme Council of the Armed Forces which took power in February and has vowed to pave the way for a democratic system.
But the armed forces, hailed as heroes at the start of the uprising for not siding with Mubarak, have come under fire from local and international rights groups for alleged abuses.
"The revolution has brought some freedom, but we need more," said Mohammed al-Sayed, 20, a student at Cairo University.
"Nothing has changed," said Mohammed Abul Makarem, 18. "Change takes time, but there are reforms we can do now."
Former Arab League chief and presidential hopeful Amr Mussa joined the mass protests, telling demonstrators he supported their "legitimate demands of speeding up the trial of the former president," who is due to face trial on August 3.
Pro-democracy groups who called for the protest were in charge of security at the entrances to Tahrir, searching anyone heading into the square and demanding to see two forms of identification.
Tarek al-Kholy, a leader of the April 6 protest movement that helped launch the January uprising, said the revolt's slogan "Change, Freedom, Social Justice" still applies.
"We want the cleansing of all state institutions of former regime members, including the universities and the judiciary. We want a reform of the interior ministry," Kholy told state TV.
"Five months after the ouster of Mubarak, we have not achieved our goals."
In Egypt's second city Alexandria, on the Mediterranean, thousands turned out to denounce the military council, calling for swift political reform.
In the canal city of Suez, at least 10,000 protesters took to the streets, an AFP correspondent said.
One man, who protesters said was a "hired thug" loyal to the former regime, fired shots into the air, injuring one person and causing momentary panic.
He was detained by protesters and handed over to security forces.
In Ismailiya, thousands chanted "Down, down with the field marshal," some holding placards demanding the end of military trials of civilians.
And in the Red Sea town of Sharm el-Sheikh, hundreds protested outside the hospital where Mubarak is in custody, demanding his swift trial and his departure from the tourist mecca that has seen little business since the January 25 uprising.
"Get lost so that Sharm el-Sheikh can get back to work," they chanted amid a heavy security presence.
The powerful Muslim Brotherhood at first dissociated itself from the rallies, but announced at the last minute that it would attend.
But the smaller Gamaa Islamiya said its members would stay away.
"The goal right now is to transfer power from the military to a civilian government. We do not support any act that will delay that process," Assem Abdel Maged told state TV.
Among the key demands at Friday's protests are ending military trials of civilians, sacking and trying police officers accused of killing protesters and the thorough and transparent trials of former regime officials.
Activists have repeatedly denounced the handling of legal proceedings against security forces who used violence in the uprising that killed 846 civilians.
On Wednesday, the government urged demonstrators to "maintain the peaceful nature of the protest," warning against "plots aiming to incite chaos in order to tarnish the country's image."
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