The state prosecutor's office accused Ms. Tymoshenko of abuse of office in causing losses of $8.5 million to the Treasury during the procurement of vehicles for use as ambulances in 2009. Prosecutors said Ms. Tymoshenko had exceeded her power as prime minister by importing 1,000 Opel Combo vehicles, which weren't designed for medical use.

Authorities have said the vehicles were used for political campaigning during Ms. Tymoshenko's run for the presidency last year. She lost in a run-off to Viktor Yanukovych.

She had already been charged with abuse of office in a separate case. She denies the allegations in both cases.

"They have fabricated another case against me," Ms. Tymoshenko told reporters Thursday as she left the prosecutor general's office in Kiev. "It's a criminal move," she added, repeating claims that the investigations amounted to "political repression."

Ms. Tymoshenko, who held office from 2007 to 2010, has come under increasing pressure from Mr. Yanukovych since he took office as president last February and then ousted her from the government.

Mr. Yanukovych has since engineered a constitutional change enabling him to fire the prime minister and other Cabinet officials.

He has further consolidated his power by appointing allies to head the security service and prosecutor's office. Critics have accused him and his allies of intimidating opposition groups and overseeing attacks on press freedom, contentions his administration denies.

Ms. Tymoshenko has been called in for questioning more than 15 times since the start of December. In the other case, she is accused of misspending $520 million in government funds allocated for environmental projects.

U.S. and European Union officials have raised concerns that investigations of opposition politicians in Ukraine are selective and politically motivated. A former economy minister was granted political asylum in the Czech Republic this month after Ukraine requested his extradition on charges of abuse of power.

Valeriy Khoroshkovskiy, Ukraine's security service chief, has denied any political motivation to the investigations. "We haven't got a separate legal code for members of the opposition," he said in a recent interview. He played down Western criticism of investigations of opposition figures, saying he saw it as a call for dialogue.

Mr. Khoroshkovskiy said the government was committed to fighting corruption, noting the arrest last year of a deputy minister from the current government, who is still being detained.

Speaking in parliament recently, Interior Minister Anatoliy Mohyliov said criminal cases had been launched against 115 members of the pro-Yanukovich Party of Regions and 73 against members of Ms. Tymoshenko's party.

Mr. Khoroshkovsky, a close ally of Mr. Yanukovych, said Ms. Tymoshenko was complaining of attacks on democratic freedoms to distract attention from her alleged wrongdoing.

Political observers are uncertain whether the authorities will jail Ms. Tymoshenko. Such a move would likely stiffen Western reproaches of Mr. Yanukovych's government, which has declared its commitment to integration into the EU.

"Jailing Tymoshenko would be received very badly in the West, so it wouldn't be a rational step," said a Western diplomat. "But they don't always act rationally."