"I saw the woman and then dozens of men surrounded her and started grabbing her, when she screamed for help some people came, but they were hit in the face," one witness said.
The woman, whose identity and nationality were not reported, was taken to a hospital after being assaulted for 10 minutes Wednesday, Biykamasr.com reported.
The number of reports of sexual harassment toward women grew throughout the day.
Activists called the attacks "unacceptable" and demanded an end to all violence toward women.
"What happened in Tahrir today has no justification and must be fully exposed even if it taints Tahrir!" wrote EgyptSecularist on Twitter.