Friends of Tarek Loubani and John Greyson said the men were told Sunday that they would be held for another 15 days, adding that no reasons were provided by Egyptian authorities.
They've been held for a full month without any charges being laid.
"We are obviously extremely saddened to hear about the extension," Cecilia Greyson, John Greyson's sister, said in a release.
"But just as importantly we are now certain that John and Tarek's detention is completely arbitrary, and the process they face lacks any semblance of fairness or credibility."
Loubani, a doctor from London, Ont., and Greyson, a Toronto university professor and filmmaker, were detained Aug. 16 during deadly riots in Cairo.
Their families say the men had approached Egyptian police seeking directions.
The two were en route to Gaza, where Loubani planned to teach emergency-room medicine and Greyson was considering producing a documentary.
Canadian foreign affairs officials have said it appears the men were in the wrong place at the wrong time when they were detained.
Their case has attracted some high-profile support. Some 300 directors, actors and writers at the Toronto International Film Festival — including Atom Agoyan, Michael Ondaatje and Sarah Polley — released an open letter last week urging their release.
On the weekend, Loubani and Greyson were moved to a less crowded cell after spending four weeks housed with 38 inmates and a single toilet, said Loubani's brother, Mohammed Loubani.
"Tarek and John have answered every question that has been put to them and now it is time for the Egyptian authorities to do the same," said Mohammed Loubani.
Lynne Yellich, the Conservative government's secretary of state for foreign and consular affairs, issued a statement saying the government is "disappointed" to hear of the extended detention.
"Canada shares the concern of the families and friends of Dr. Loubani and Mr. Greyson in this matter and, in the absence of confirmation of the charges, continues to call for their release," Yellich said in the release.
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