Paris-based media watchdog Reporters Without Borders (RSF) says Mirsayafi's death could have been avoided if prison staff had not been negligent and had acted promptly.
"His death is all the more regrettable as his detention was totally unjustified," RSF says in a statement to mark the occasion.
Mirsayafi was serving a two-year prison term after his conviction on charges of insulting Iranian leaders in his blog when he collapsed on March 18, 2011. It reportedly took prison authorities three hours to get him to a hospital, where he later died. Officials told the family that Mirsayafi committed suicide by overdosing on sedatives. But the family cast doubt on the official version and called for a second autopsy -- a request that authorities rejected.
RSF says it supports the family's call for an investigation by an impartial commission and the release of all documents that could shed light on the circumstances of Mirsayafi's imprisonment and death.
The group calls Mirsayafi's death emblematic of the tragedy affecting dissidents and critics in Iran today:
"He is a symbol of all those who are hounded, persecuted, arrested, jailed and silenced. We pay tribute to his memory and we demand the immediate release of the 50 or so journalists and bloggers who are currently detained in Iran just for expressing their views freely."
The March 18 Movement, an online movement created in the wake of Mirsayafi's death, has declared March 18 as "a day of memorial to Omid and to all the innocents he represents."