By Helen Fawkes
BBC News in western Ukraine
Ukraine's opposition leader Yulia Tymoshenko has said she believes that the early parliamentary ballot due to be held in September will be rigged.
Mrs Tymoshenko has fears over electoral legislation
Election officials have already tried to bar her party from running.
Mrs Tymoshenko, along with President Viktor Yushchenko, helped to lead the country's 2004 Orange Revolution, which was sparked because of vote-rigging.
This election was called after a power struggle between the president and Prime Minister Viktor Yanukovych.
The start of her campaign was delayed because election officials appointed by the prime minister refused to allow her party to take part in the ballot, until they were ordered to do so by a court.
Mrs Tymoshenko said that she was worried that the election would not be free and fair because laws which were introduced to try to stop vote rigging were recently reversed by election officials.
She said: "It seems to me that the country has started moving backwards in respect to its election legislation."
International observers have already expressed concern that next month's vote could be open to fraud.
She added: "After the changes that the prime minister's team made to the election rules, Ukraine is again facing the threat of massive falsification."
But Mr Yanukovych has denied that his party has tried to rig the ballot.
It is running a high profile campaign and the opinion polls all predict that the prime minister's party will win the largest share of the vote.