I'm convinced now that millions of Iranians are convinced of voter fraud. Since there were no international observers, that's hard to prove either way.
If Iran doesn't hold a new election, people who believe its now impossible to change the government by peaceful democracy will go underground and begin to plot the overthrow of the entire system. Typically that leads to violence and revolution. Iran's religious leaders should be aware of this danger. After all that how they overthrew the previous US/British installed dictator.
The election has little effect on the power of Iran's supreme leaader. But these demonstrations threaten his power base and legitimacy. If these demonstration continue to grow, he faces a choice:
Endorse a percieved rigged election, massive protests, violence, bloodshed and possible revolution
or
Holding new elections which will legitimize his position as supreme leader
I suspect he will choose new elections.