Citizen's arrest based on what? You need to tread carefully when doing such a thing.
No, it's simple as pie. If you have grounds to believe that there is a crime going on, and that the criminal may be about to run with the money, you just detain him and leave the matter to the police for investigation. The police cannot fine you unless they find out that Mt.Gox is committing nothing criminal. In the complaint you outline the crimes in such technical precision that the police cannot find them not true without real investigation. So they will have to investigate.
Of course they
can summarily decide that you are wrong and the arrest was unlawful, but if Mt.Gox ever is found guilty (which I would say is 99% sure at this point), you will triumph as a hero and Japan police and judicial system is shamed.
This is exactly why citizen's arrest prevails in the legal code. When it's taken away, then there is nothing more to do but whine.so we need at least four citizens arresting the weighty magicien before he is walking through the magic wand...
or better wait until Mr. Sturle confirms the arrival of his SEPA payment ?