Muslim prayers also resounded in the square "in what seemed a show of interfaith harmony" five weeks after a suicide bomber killed at least 21 people at the end of a New Year's Eve mass in Alexandria, The New York Times reported.
"We are all one," people began chanting in Tahrir Square after the outdoor Coptic Christian mass was completed.
State television has asserted most protesters seeking political change were part of the Muslim Brotherhood, and some members of the Coptic minority have accused their leaders of reluctance to take stands for political change that the government could see as confronting the state, the Times said.
Sunday's mass was "for all Egyptians, Muslim and Christian, and I am proud to be Egyptian today because we are showing the world how important our country is for all the people who live here," a 33-year-old Christian identified as Farid told the Egyptian news Web site Bikya Masr after the liturgy was completed.
"What we witnessed today is the unity of Egypt for a common cause," Farid said.
Activists on the online Twitter microblogging service also called for unity and declared the mass was "for all Egyptians."
A photograph posted on Facebook by the global Humanity's Team movement showed Egyptian "Christian protesters protecting Muslim fellows during their prayers" in Tahrir Square.
One activist said he hoped government and opposition negotiators meeting Sunday would make oneness a foundational principle of the new government and then apply that principle to everyday life.