Ukraine's presidential election is a "decisive signal of their support for reform", William Hague declared as he praised voters for defying disruption by pro-Russian separatists.
The Foreign Secretary said every vote cast was "an individual act of courage" as he pledged to work with the country's new leader on ambitious reforms.
Exit polls suggested a convincing win for billionaire chocolate tycoon Petro Poroshenko, who is said to have received 55.9% - with former prime minister Yulia Tymoshenko a distant second.
Voting was severely disrupted in parts of the east of the country, where separatists have seized control amid fighting that has claimed at least 20 lives in recent days.
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West Yorkshire Police confirmed a 32-year-old woman was taken into custody in connection with the alleged threats.