Friends and former colleagues and students of a jailed Canadian-Iranian academic still can't believe she's behind bars in Iran, several of them said Wednesday at a rally calling for her release.

Many at the event outside Concordia University in downtown Montreal held signs and wore light blue T-shirts bearing Homa Hoodfar's image and the hashtag #freeHoma.

Concordia professor Margie Mendell said she couldn't imagine her close friend in the depths of Tehran's notorious Evin prison.

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"She's an exceptional person in every way," she said. "She's brilliant, she's lovely, she's delightful company, she's warm, she's generous, and one of the most open-minded people I ever met."

Hoodfar, 65, has been imprisoned since June 6 on what her friends and family describe as trumped-up charges of collaboration with a hostile government and propaganda against the state.

Her family has said the Iranian probe into the retired anthropology professor centred on her "dabbling in feminism" and security matters.

Mendell said Hoodfar's captors must have "completely misunderstood" her academic work, which focused on Muslim women in various regions of the world.

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"Homa is so balanced, so reasonable in her approach," she said. "She's so committed to respecting and understanding diversity and difference and not universalizing our perspectives of how we see people."

Many at the rally described Hoodfar as a warm, funny person who became a mentor to many of her students.

"It's really scary that someone who's contributed so much and has added so much to the world around her and her communities could find herself in such a horrifying situation," said Hayley Lewis, a former student and one of the event's organizers.

Many in attendance said they were worried by reports that Hoodfar had to be transported to hospital because of her rapidly declining health.

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Hoodfar suffers from a serious neurological condition and her family has said requests for a check-up by an independent specialist doctor have been ignored.

"She is sick and she is unwell and we need her back," Lewis said. "We can't afford to let her disappear."

Wednesday's rally follows another that was held outside the Iranian Embassy in Dublin earlier this month.

Hoodfar also holds Irish citizenship.

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Some 5,000 academics worldwide have also signed a petition in support of Hoodfar.