Parliament approves legislation needed to hold elections on September 30.
Her party, Yushchenko’s Our Ukraine and People’s Self-Defence party, and Yanukovych’s Regions party, are expected to capture most of Sunday’s vote.
But none of the three parties is likely to win a clear majority in the Rada, or parliament, meaning coalition talks are inevitable.
If the Western-leaning Tymoshenko and Yushchenko do join up, expectations are that Tymoshenko would seek to become prime minister, forcing pro-Moscow Yanukovych out of government and into opposition.
Yanukovych has also wooed Yushchenko, saying that they could form a “broad coalition” in which he would retain the premiership.
Sunday’s election was called after a power struggle between Yanukovych and Yushchenko, who wants Ukraine to join Nato and the European Union.
Tymoshenko used to serve as prime minister but was sacked after falling out with Yushchenko, her former ally in the “Orange Revolution”.
Police said about 10,000 people attended Tymoshenko’s demonstration, where her supporters waved the party’s red love-heart flag and placards inscribed: “Yulia for prime minister”.