More than 100 people set up a tree and decorated it, the press office of her Batkivshchyna (Fatherland) Party said. At midnight, they lit sparklers.
A spokeswoman said the party hopes Tymoshenko will be free by the end of 2013, Russian news agency RIA Novosti reported.
After she left office, Tymoshenko was convicted of abuse of office for a gas deal she negotiated with Russia and sentenced to seven years. Critics of the government said the trial and punishment were political.