"Muslims and Christians are one," chanted flag-waving protesters in the swelling crowd, as others held up banners depicting an entwined cross and crescent.
The rally comes a day after Egypt's new military rulers met with representatives of Christian Copts in a bid to allay fears of a return to sectarian violence.
Bloody fighting erupted on Tuesday in the working class Cairo district of Moqattam when Muslims confronted 1 000 Christians who had been blocking a main road in protest at the burning of a church last week.
The health ministry said 13 people were killed in the clashes, and Father Boutros Roshdy of the Moqattam church said at least seven Copts were among the dead.
Hundreds of Copts continued days of demonstrations on Thursday outside state television headquarters in Cairo to demand the rebuilding of the church torched last week in the provincial town of Sol, south of the capital.
The clashes there were sparked by a romantic liaison between a young Christian and Muslim that left two people dead.
Egypt's youth coalition, which launched the revolt that forced strongman Hosni Mubarak to step down on February 11, had called on its Facebook page for a peaceful demonstration in Tahrir Square on Friday for "national unity" between Egyptians.
The group urged people to "prevent any attempt to provoke dissent or chaos".
Attackers armed with knives and machetes waded into hundreds of pro-democracy activists in the square on Wednesday.
The violence, widely blamed on remnants of Mubarak's regime, revealed the security vacuum created by police, who disappeared from the streets during January protests that led to Mubarak's resignation.
Newly appointed interior minister Mansour Essawy vowed police would be back on the streets in their "full capacity", amid efforts to restore security in the city.