Canada has fingered the former Tehran prosecutor as bearing responsibility for the death of Canadian-Iranian journalist Zahra Kazemi in 2003.
"We are deeply concerned by reports that Saaed Mortazavi has been put in charge of the investigation of detained reformist leaders and party officials in Iran," Cannon said in a statement.
"On several occasions, most notably the detention and murder of Canadian journalist Zahra Kazemi, Mr Mortazavi has clearly demonstrated his disregard for human rights and fundamental freedoms," he said.
Cannon also said Iran's own probes confirmed that Mortazavi ordered Kazemi's "illegal" arrest and detention, and that he "falsified documents to cover up his involvement in her case."
Kazemi, 54, was detained in June 2003 for photographing a demonstration outside Tehran's notorious Evan prison. She was beaten in custody, and died from a cerebral haemorrhage after more than two weeks under arrest.
The case contributed to a souring of relations between Iran and Canada.
In 2006, Canadian Prime Minister Stephen Harper called on the international community to use "all manner of law available" to bring Mortazavi to justice over Kazemi's death.
To this day, Ottawa continues to press Tehran "to conduct a credible investigation that will bring justice to those responsible for the murder of Ms Kazemi."
