Security forces are surrounding Russian facilities in Ukraine amid anger over the Ukrainian government's refusal to allow ordinary Russians to vote for president.
Ukrainian police are guarding the Russian Embassy in Kiev and consular offices in Odessa and other cities.
The Ukrainian government announced that only Russian diplomatic officials would be allowed to cast ballots in Sunday's vote, which Vladimir Putin is set to win.
Millions of ethnic Russians live in Ukraine but the number of registered Russian voters in Ukraine is unclear.
Ukraine is protesting voting in Crimea, annexed by Russia from Ukraine four years ago. Ukraine is also angry over Russian support for separatists in eastern Ukraine, where a deadly conflict continues.
Russian authorities are appealing to the United Nations and Council of Europe to intervene, according to Russian news agencies.
Russian President Vladimir Putin has cast his ballot in the presidential election, seeking a mandate for a fourth term.
Putin is certain to win in Sunday's election, so voter apathy is widespread. Authorities have spent unprecedented funds to get out the vote to ensure he has a strong mandate for his next six years in office.
Some 145,000 observers are monitoring the voting in the world's largest country, including 1,500 foreigners and representatives from opposition leader Alexei Navalny's political movement.
Navalny himself is barred from running. Putin faces seven challengers but none poses a serious threat.