Mahfouz, a well known youth leader, was a leading figure of the uprising that toppled former president Hosni Mubarak in early 2011.
She was stopped at Cairo airport by immigration officials following a request from the prosecutor general’s office, an airport official said, adding that her name was on a list of people barred from leaving the country.
After briefly detaining her, she was told to go back home, the official said.
Mahfouz was attempting to travel to Bangkok with her sister and sister’s husband.
Since the ouster of Islamist president Mohamed Morsi last year, the authorities have cracked down on opposition, including pro-Morsi Islamists and secular activists.
Several secular activists have been detained, jailed or barred from leaving the country, while the crackdown on Morsi’s supporters has left at least 1,400 people dead, and over 15,000 arrested.