Russia could gain as many as 10 million new outlaws by Oct. 5, once the deadline has passed for dual citizens to declare their status as such to the Federal Migration Service.
President Vladimir Putin signed a law in June requiring Russians with foreign passports to report their second citizenship to the government.
Foreign residency permits must also be reported, ministry official Valentina Kazakova said at a press conference in Moscow on Thursday, where the Oct. 4 deadline was announced.
Failure to register will constitute a criminal offense punishable by a fine of up to 200,000 rubles ($5,500) or up to 400 hours of community service.
Russians who live outside the country will not be required to report their status by the Oct. 4 deadline, Kazakova said. In any case, reporting can only be done at the migration service's office. Embassies and consulates are not authorized to handle the job.
But those with multiple passport or foreign residency permits will have to report themselves within 60 days of their first visit to Russia, Kazakova said.
The rule applies even if the holder of two passports pops over to the fatherland for a brief visit — say, the New Year's celebrations, she said.
Russian dual citizens that blow the deadline may be able to get away with a fine of 500-1,000 rubles ($15-30), but are strongly encouraged to register before a criminal case is launched.
At present, it remains unclear how a citizen who misses the deadline due to living abroad will be required to prove that he or she was, indeed, out of the country.
Migration officials and the migration watchdog's press service also failed to immediately clarify whether people who have previously declared their second citizenship status — e.g. in passport applications — would be required to redeclare.
Call center staff at the Federal Migration Service's Moscow office provided conflicting answers to various queries on the issue during separate calls made Thursday, though cohesion appeared to be on the rise as the day progressed.
As many as 10 million people could be affected by the law, according to estimates by the think tank Migration XXI Century in Moscow, cited by RBC Daily news website.
The Russian diaspora worldwide consists of up to 37 million people, including about 17 million living in the former Soviet republics, according to Migration XXI Century.
The dual citizenship bill was introduced by conservative lawmakers with President Vladimir Putin's blessing after Russia's relations with the West began to deteriorate over the ongoing crisis in Ukraine.
It is widely seen as being aimed at the potential "fifth column" of Russians living in the EU and the U.S.
"I guess if Catherine the Great were living here now, it would mean she'd have to register as a German citizen," quipped a Muscovite with an American passport she obtained by marriage, in reference to the empress' Teutonic heritage. The woman declined to give her name to avoid trouble with migration authorities.