Former Secretary of State Madeleine Albright is in Ukraine, leading the National Democratic Institute's election observation mission.
Albright tweeted a picture from a polling station Sunday morning.
Ukraine, which has been engulfed in turmoil with Russia, is holding presidential elections Sunday after ousted President Viktor Yanukovych fled to Russia amid protests in the country.
High turnout is reported at polling stations in Ukraine.
"These are extremely important elections. We have to make sure Ukraine becomes a truly independent country, a powerful independent state that nobody will be able to push around," pensioner Mihailo Belyk, 65, told Reuters as he voted in a southeastern district of the Kiev.
A delegation of lawmakers including Sens. Ben CardinBen CardinWeek ahead: Comey under fire; Lawmakers look for Russia response Senate heading toward late-night marathon session Rocky start for Trump's State Department nominee MORE (D-Md.) and Rob PortmanRob PortmanSchumer puts GOP on notice over ObamaCare repeal Trump, House GOP could clash over 'Buy America' US launches trade case against China over aluminum subsidies MORE (R-Ohio) is also in Ukraine to help monitor the vote.
In Ukraine observing their presidential elections. An important day for Ukrainians. pic.twitter.com/bgAUSREKul— Rob Portman (@robportman) May 25, 2014
Portman tweeted that more than 100 Ukrainians "gave their lives fighting for a free & democratic Ukraine." "Their spirit lives on" through the election, he added.
Cardin tweeted that Russia "has violated just about every agreement they've entered," adding that observers were hoping to see a "free and fair election."