“The results of this illegal vote are legally void, will not have any legal consequences and will not be recognized either by Ukraine or the international community,” the statement said.
The Foreign Ministry also called on international partners to give a “principled assessment” of Moscow’s actions and “increase pressure” on it.
Earlier, Ukrainian leader Vladimir Zelensky expressed confidence that Crimea would “return” to Ukraine.
September 8 is a single voting day in Russia.
Crimea became the Russian region after a referendum held in March 2014, in which most residents spoke out for reunification with Russia.