A court in the Ukrainian capital Kiev on Tuesday sentenced former prime minister Yulia Tymoshenko to seven years in prison.
Judge Rodion Kireyev said Tymoshenko was guilty of exceeding her authority when signing a gas deal with Russia in 2009 – and using her powers to "criminal ends”.
Prosecutors accused Tymoshenko of ignoring the best interests of the state by forcing through the gas contract.
Agence France Presse reported that Tymoshenko vowed to appeal against her sentence, and would take the case to the European Court of Human Rights.
Riot police were deployed outside the court, as supporters and opponents gathered.
According to the BBC, Tymoshenko, a former Orange Revolution leader, has argued the charges against her are politically motivated.
(Read more on GlobalPost: Yulia Tymoshenko, under fire and fired up)
Both the United States and the European Union have condemned the charges against Tymoshenko, saying Ukraine had engaged in a selective prosecution of political opponents.
Former president and one-time ally Viktor Yuchshenko testified against her, along with others.
Watch a video of the clashes that erupted after Tymoshenko's arrest: