Yulia Tymoshenko, Ukraine's defeated presidential candidate, has declared that she will challenge the election which saw her opponent, Viktor Yanukovych, win the presidential run-off.

Tymoshenko said on Saturday that she will fight the result of last week's vote in Ukrainian courts, calling Yanukovych's win illegitimate.

"I want to clearly state: Yanukovych is not our president. Whatever happens in future, he will never become the legitimately elected president of Ukraine," she said in a televised broadcast.

She also called on the Ukrainian people to support her legal battle to overturn the election result.

But she said she would not call on her supporters to take to the streets in protest.

Ukraine's central election commission said on Wednesday that Yanukovych had won 48.95 per cent of the vote in the presidential poll, compared to Tymoshenko's 45.47 per cent.

Although official, the results are still preliminary and must be formally certified by the commission this month.

Tymoshenko's team has claimed widespread violations took place during voting, particularly in Yanukovych's eastern Ukraine strongholds, but international observers hailed the vote as a fair and clean.