With two defectors from the Socialist Party allying themselves with Tymoshenko's bloc, the opposition has 123 seats in the 450-seat Ukrainian parliament.
"We want to raise the flag that was dragged through the mud by politicians' treason," Tymoshenko declared.
Prime Minister Viktor Yanukovych, whose fraud-marred bid for the presidency sparked the 2004 Orange Revolution mass protests, put together a majority coalition this summer after an agreement between Tymoshenko and her former Orange Revolution allies fell apart.
The ruling coalition, which also includes the Communists, nominated Yanukovych to be premier, and President Viktor Yushchenko agreed to it, after making Yanukovych pledge to uphold the country's democratic advances and continue his pro-Western policies.
Yanukovych, who had the Kremlin's backing in his failed 2004 presidential bid, is seen as more pro-Russian.
During his recent visit to Brussels, Yanukovych said that Ukraine was putting its bid to join NATO on hold because of widespread public opposition in the country — a move that was sharply criticized by Yushchenko.
Yanukovych has said that Ukraine's bid to join the European Union remains on target, and he has taken the West's advice and ruled out joining a customs union with Russia and other ex-Soviet republics.
Tymoshenko, whose fiery speeches helped make her a leader of the Orange Revolution, had hoped members of Yushchenko's pro-Western party would join her in opposition, but they accepted seats in Yanukovych's government and are still holding talks on joining Yanukovych's coalition.
She has called Yanukovych illegitimate and demanded the dismissal of parliament, but she is far short of the support necessary for such an action.
"The ideas of democracy and European choice are our key priorities," said lawmaker Iosyp Vinsky, one of two Socialist Party members who joined Tymoshenko's bloc in creating the new opposition.