The FreeRoxana campaign began in an effort to pressure Iran to release the 1999 graduate of the university's Medill School of Journalism. Volunteers around the world are asked to fast in 24-hour shifts.
The North Dakota native was arrested in January and convicted last month. She was sentenced to eight years in prison after a one-day trial behind closed doors.
Saberi began her own hunger strike April 21.
Northwestern University student David Caratelli says the hunger strike is necessary "to create awareness about the situation Roxana is facing."
The American journalist on a hunger strike for two weeks herself is protesting her imprisonment in Iran. Saberi was briefly hospitalized after she intensified her fast by refusing to drink water, Reporters Without Borders said Monday.
The press freedom group said 32-year-old Saberi was taken Friday to a clinic at Tehran's Evin prison, where she has been held since her arrest in January. She was released from the clinic within a day after again drinking water, the group said.
Saberi's Iranian-born father, who traveled to Iran to seek his daughter's release, said last month that she was drinking only sweetened water while refusing food to protest her eight-year jail sentence.
Reporters Without Borders said her father, Reza Saberi, told the group over the weekend that she stopped drinking water after Iranian authorities denied she was on a hunger strike.
"So following that, she decided to do a complete hunger strike," Soazig Dollet of the Paris-based group told The Associated Press. "So she was really weak and went to the clinic inside the prison for the day, but not more than a day."
Saberi's father did not answer phone calls seeking comment on Monday.
The Obama administration has called the espionage allegations against Saberi baseless and demanded her immediate release. Iranian authorities have promised a fair review of her appeal.
Saberi, a dual Iranian-American citizen, has lived in Iran for the last six years. She was born in the U.S. and grew up in Fargo, North Dakota.
She was initially accused of working without press credentials, but authorities later made the more serious charge that she passed intelligence to the U.S. She was convicted on the espionage charge after a one-day trial behind closed doors.
The case has been a source of tension between the U.S. and Iran at a time when the Obama administration is reaching out to Tehran after decades of diplomatic stalemate.
Saberi was working as a freelance reporter for organizations including National Public Radio and the British Broadcasting Corp. before her arrest.
In Tehran on Monday, Iran's Foreign Ministry spokesman called for an end to what he called outside interference in the case from groups like Reporters Without Borders.
Four of the journalist group's members, including its secretary-general, began their own hunger strikes a week ago in support of Saberi, while urging her to end her own protest out of concern for her health.
Ministry spokesman Hasan Qashqavi said the group's protest was not welcome.
"Iran's judiciary is an independent body and any foreign attempt to intervene in it goes against international measures," he told reporters in his weekly briefing.
"This is not a complicated issue. This Iranian woman has been sentenced and should wait and see what verdict the appeals court will issue," he said.
The Associated Press contributed to this report.