"Unfortunately, there will be no time for me to take a rest," she said at a news conference.

She said she would head the opposition and her eponymous Yulia Tymoshenko Bloc.

Earlier in the day the Ukrainian parliament formed a majority parliamentary coalition loyal to President Viktor Yanukovych and appointed a new prime minister, ending a political stand-off between the legislature and the head of state.

Tymoshenko advised Yanukovych not to relax.

"He will have to give an account to society every day for his unconstitutional actions," she said.

On Tuesday, Tymoshenko was elected the leader of the "united democratic opposition." She urged the country's democratic forces to rally against Ukraine's new authorities.

Tymoshenko lost the February 7 presidential runoff to Yanukovych, who was sworn in as president on February 25.

On March 3, following a collapse of the majority coalition led by Tymoshenko, the country's parliament voted to oust her government, improving the chances of Yanukovych's Party of Regions to form a new parliamentary majority he needs to push through his policies.

Tymoshenko then announced that her party would move into the opposition without waiting for the formation of a new majority coalition.