European leaders and U.S. President Barack Obama have dismissed the vote, which has been organised by Crimea's pro-Russian authorities at short notice, as illegitimate, saying it would violate Ukraine's constitution.
According to ballot papers published before the referendum, voters have the right to choose one of two options, neither of which rejects control by Russia.
The first question asks: "Are you in favour of the reunification of Crimea with Russia as a part of the Russian Federation?"
The second asks: "Are you in favour of restoring the 1992 Constitution and the status of Crimea as a part of Ukraine?"
At first glance, the second option seems to offer the prospects of the peninsula remaining within Ukraine. But the 1992 national blueprint is far from doing that.
Instead, it foresees giving Crimea all the qualities of an independent entity within Ukraine - but with the broad right to determine its own path and choose relations with whom it wants - including with Russia.
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