KYIV (Reuters) - The Belarus prosecutor’s office has opened a criminal case against opposition leader Sviatlana Tsikhanouskaya on suspicion of preparing a terrorist act, the Belta news agency said on Monday, citing prosecutor general Andrei Shved. Tsikhanouskaya fled to neighbouring Lithuania shortly after a disputed Aug. 9 election and called for President Alexander Lukashenko to leave power. Lukashenko, 66, has faced the biggest challenge to his nearly 27-year-rule since protesters took to the streets after he was declared the winner of an election they said was rigged. Belarusian authorities said last week they had arrested a man who, according to them, was preparing an explosion in Minsk and at a military unit near the town of Borisov. Prosecutors accused the opposition of involvement in the incident, which the opposition denied. On Monday, Shved said Tsikhanouskaya was also involved in preparation of the terrorist attacks, but gave no details. Prosecutors said they would demand the extradition of Tsikhanouskaya, who was not immediately available for comment.