Some polling stations will be closed and staff pulled off amid an "atmosphere of terror" building in Ukraine ahead of the country's presidential elections.
Europe's OSCE democracy watchdog has said most of its foreign election monitors would not be working due to fears for their security.
Polling stations in the eastern region of Donetsk and Luhansk are unlikely to open.
The Vienna-based body has over 1,000 vote monitors in Ukraine for the election and is expected to deliver a verdict on the conduct of the poll on Monday after results are announced.
Kiev hopes the poll will help resolve a confrontation with Russia that has split the country.
If necessary, a second and final round of elections will be held on 15 June.
More top news
The Duke of Cambridge became a fearsome samurai warlord - at least temporarily - when he visited the set of a long-running Japanese drama.
A grandmother says she was outraged after her grandson was segregated from his classmates by a cardboard screen - for being one minute late.
Leaked documents have revealed the first known picture of Mohammed Emwazi - named as IS militant Jihadi John - as a student in London.