Tymoshenko could face up to several years in prison and is expected to be sentenced later Tuesday.
Authorities deployed hundreds of police officers around the court to keep order, state media reported. Dozens of angry Tymoshenko supporters took to the streets of Kiev in August when she was taken into custody.
In April, the Ukraine's Prosecutor General's office opened a criminal case charging Tymoshenko with signing overpriced gas deals with Russian energy provider Gazprom that inflicted damages to the country of more than 1.5 billion hryvnas (almost $190 million at the current exchange rate) and which Tymoshenko had allegedly no right to sign.
Tymoshenko repeatedly brushed off all charges against her as political, calling the trial a "farce" and naming the judge a "stooge of (President Viktor) Yanukovych's administration" appointed to "fabricate" the case.
The Russian foreign ministry said the nation's 2009 natural gas agreements with Ukraine were legal.
"We follow the principle that (Yulia) Tymoshenko's trial must be fair and unbiased and meet all of the requirements of Ukrainian legislation, with the possibility of defense and compliance with the fundamental humanitarian standards and rules," according to a ministry statement.
Tymoshenko narrowly lost to Yanukovych in a presidential election in February 2010, and became his fiercest opponent.
