Mousavi's charge that the elections, in which Ahmadinejad claimed victory early on, were fraught with corruption and fraud garnered sympathy from leaders around the world including U.S. Vice President Joe Biden.
Mousavi seems to have the backing of a large network of young and middle-class voters and could emerge as Iran's liberal voice to put pressure on Ahmadinejad's policies and confrontational tactics with the West.
French Foreign Minister Bernard Kouchner said following Ahmadinejad's election that he was "very worried" about the situation in Iran and criticized what he called the "somewhat brutual recation" to the post-election protests.
Biden said today on NBC's "Meet the Press" that "we just don't know enough" about the happenings in Iran since Friday's vote.
"Is this the result of the Iranian people's wishes?" he asked. "The hope is that the Iranian people, all their votes have been counted, they've been counted fairly."
The Associated Press contributed to this story.