Ukrainian officials have slammed Russia for holding parliamentary elections in Crimea, a peninsula annexed by Moscow in 2014.
Pro-Russian separatists in the territory can attend polling stations and participate in this weekend's vote.
Moscow, which considers Crimea part of its territory, has also distributed more than 600,000 passports in east Ukraine, where citizens can vote electronically or in the Russian border region of Rostov.
But Ukrainian foreign minister Dmytro Kuleba said Russia was committing a "flagrant violation" of international law.
Calling on the United Nations to help "restore the territorial integrity of our state", he told reporters: "The Russian occupation will end."
Kyiv has also accused Moscow of speeding up the issuance of passports in separatist territories to boost participation in the election.
The Ukrainian parliament has also declared Russia holding elections in Crimea "illegal" and urged the international community not to recognise the result.
Since the annexation of Crimea, Ukraine has been in conflict with pro-Russian separatists who are widely believed to be supported by Moscow. More than 13,000 people are known to have been killed in the conflict.