Her son, Stephan Hachemi of Montreal, says she was accused of being a spy. Canadian Foreign Affairs officials informed him of her death early Saturday morning.
Hachemi told reporters that his only consolation was that his mother, a freelance photojournalist of Iranian origin, died doing what she believed in.
"She was a woman, she was making a difference, she was making a change and that's very important," the 26-year-old Hachemi said.
Foreign Affairs Minister Bill Graham is demanding a a complete explanation from Iranian authorities of the circumstances of her death. A meeting in Tehran could come as early as Sunday.
Canadian embassy officials had seen Kazemi every day between Monday and Thursday, according to Foreign Affairs department official Reynald Doiron. Their latest visit was Thursday, when they also met with the Iranian doctors treating her.
Kazemi's family alleges the photographer suffered a brain injury during a violent police interrogation.
The Canadian Embassy in Tehran learned of the case after Kazemi called her mother, who lives in the southern Iran town of Shiraz, to say she had been detained in Tehran. Kazemi's mother then contacted Canadian officials with the story.
"She was beaten up and went into a coma two weeks ago, after the phone call," Hachemi said.
Kazemi has worked for the Montreal-based magazine Recto Verso on a freelance basis for about seven years. Hachemi said he last heard from his mother -- who spent 1½ months in Iraq after the U.S.-led war there was over before travelling to Iran -- on June 23, the day before her reported arrest.
In her last email to her son, Kazemi described the anti-government student protests that engulfed the capital, sparking mass roundups and the detention of numerous journalists by security forces.
"The country is living through nighttime upheavals that are ideal for photographers," Kazemi wrote.
Reporters Without Borders said more than half a dozen reporters have been arrested in Iran following a security clampdown after student protests in recent weeks. They're calling for an independent investigation into the death of Kazemi.
"For us, the most important thing is to have the body back in Canada. Without that, we cannot do an autopsy to see how she died, or why she died," Tanya Churchmuch, president of the Canadian chapter of the organization, told a news conference.
"But we cannot get the body back without the consent of the Iranian government."'
With a report from The Canadian Press