The Muslim Brotherhood, Salafists and other revolutionary groups called for the protest in Tahrir Square Friday against Egypt's ruling Supreme Council of Armed Forces, Ahram Online reported.
Protesters carried banners and pictures of the Muslim Brotherhood's presidential candidate, Mohamed Morsi, who they said was the winner of the country's recent run-off election.
The protest was organized after the election committee overseeing the voting process delayed announcing the results of the election Wednesday, saying it had to investigate voter-fraud claims, Bikyamasr.com reported.
"We are here to end the military rule over the country and make sure Morsi is elected as the next president," said Brotherhood supporter and youth activist Hossam Marwan, who told Bikyamasr.com Friday will be the "most important day for the revolution."
Sheik Mazhar Shahin, the imam of Omar Makram Mosque, delivered the Friday sermon at the protest, the Middle East News Agency reported.
"The battle was and still is over the rights and dignity of the people. We won't allow the return of the former regime after the revolution toppled it," Shahin said. "We demand there be no manipulation of the presidential election results. Tahrir Square wants legitimacy and legitimacy is on the side of Mohamed Morsi."
Meanwhile, the SCAF said the Muslim Brotherhood's early claim of Morsi's victory can cause political confusion, Bikyamasr.com reported.
"Prematurely announcing the results of the elections is the reason behind the political confusion we are going through," said a statement from the SCAF.
The SCAF's statement also said upholding state institutions is the responsibility of all citizens and said not doing so could threaten the security of the country, Egypt Independent reported.
"The rule of law is the basis of governance in the state," the statement added.