Mir Hussein Mousavi has issued a letter objecting to the results
. It looks rather toothless, however, as he doesn't say much about what he plans to do about them. One problem with reformists during the Khatami years was their fear of street confrontations and belief they had to get what they wanted by playing according to the rules to stave off revolutionary chaos. That's hard enough to do when the rules are rigged against you, much less when the other side has abandoned them completely.
UPDATE: Karroubi is vowing not to concede