The campaign follows a call by dissident journalist Akbar Ganji for protests that call attention to the plight of prisoners of conscience in Iran.
Activists in London, New York, and Cologne say they are participating in hunger strikes.
Hojat Sharifi, a leader of Iran's largest reformist student group, told RFE/RL's Radio Farda that "more than 100 people" have come to Tehran and "expressed their support."
"Student activists in other cities, such as Hamedan, also are joining the hunger strike," said Sharifi, who is himself on a hunger strike.
Iranian officials have repeatedly denied that there are any political prisoners in the country.