KIEV, Ukraine (AP) — Former Ukrainian Prime Minister Yulia Tymoshenko has announced her presidential bid.

Opinion polls have placed the 58-year-old, who was the country's prime minister between 2007 and 2010, as a front-runner in the presidential election set for March 31.

Tymoshenko spent two-and-a-half years in prison for signing a gas deal with Russia, which was largely viewed as retribution by her political rival, then-President Viktor Yanukovych. Tymoshenko lost to Yanukovych in the 2010 presidential vote and to the current incumbent, Petro Poroshenko, in 2014. Poroshenko has not formally announced his bid for re-election yet.

Tymoshenko announced her bid at her party's congress in Kiev on Tuesday, promising to regain control of the Russian-annexed Crimean peninsula and the separatist-held areas in the east.