BBC News charts how the crisis evolved.

23 January: Viktor Yushchenko is sworn in as Ukraine's new president ending the bruising election marathon. In taking the oath of office before parliament, Mr Yushchenko said he would defend the unity of Ukraine.

20 January: Supreme Court rejects Mr Yanukovych's final appeal against the result of the re-run election and declares Mr Yushchenko the winner. Parliament votes to hold Mr Yushchenko's inauguration on 23 January.

Mr Yanukovych concedes he has lost the election re-run to Mr Yushchenko, telling supporters in his Donetsk stronghold: "The right of force has won against the force of the law."

18 January: A ban on publication of the presidential election results is lifted by the Supreme Court - allowing them to be published in newspapers on 20 January, making them legal.

17 January: Supreme Court starts hearing Mr Yanukovych's final appeal after he submitted 600 volumes of evidence indicating irregularities in the re-run election. All his previous appeals have been rejected.

Yanukovych and Yushchenko had to wait nearly three months for a result

16 January: Thousands of demonstrators rally in Mr Yanukovych's home town, Donetsk, and elsewhere to condemn Mr Yushchenko's "anti-constitutional" election.

11 January: Electoral commission declares Mr Yushchenko the official winner of the re-run presidential election with 51.99% of the vote. Mr Yanukovych gets 44.2%, but continues the legal battle.

6 January: Supreme Court rejects an appeal by Mr Yanukovych against the electoral commission's handling of the poll. The ex-prime minister had wanted the court to make the commission re-examine complaints about the election.

5 January: Outgoing President Leonid Kuchma accepts Mr Yanukovych's resignation as prime minister.

31 December: Mr Yanukovych resigns as prime minister, saying he cannot work with people loyal to Mr Yushchenko.

30 December: Supreme Court rejects all four complaints against the conduct of the presidential election lodged by Mr Yanukovych. The Central Election Commission also rejects his appeal over the vote.

27 December: With nearly all votes counted Mr Yushchenko's lead becomes unassailable, but Mr Yanukovych says he will never concede defeat, claiming election abuses.

26 December: The third and decisive round of the presidential poll proceeds smoothly. As the counting begins, it appears that Mr Yushchenko is set to win.

25 December: Constitutional Court strikes down reform restricting home voting; election officials say vote will proceed regardless.

24 December: Campaigning ends at midnight, with both candidates saying they are confident of victory.

20 December: The two rivals accuse each other of electoral fraud, in a live TV debate ahead of the poll re-run.

11 December: Mr Yushchenko's Vienna doctors confirm after exhaustive tests that he was poisoned with a form of deadly dioxin.

9 December: Government employees return to work after opposition demonstrators scale down their protest in Kiev.

8 December: Parliament passes a wide-ranging reform bill, paving the way for a 26 December re-run of the disputed presidential election.

3 December: The Supreme Court annuls the results of the second round of the elections, paving the way for fresh elections.

2 December: Crisis talks to try to find a solution to the deadlock continue as parties await the decision of the Supreme Court.

1 December: Parliament narrowly passes a motion of no-confidence in the government on the second attempt, prompting opposition fireworks in Kiev, but Mr Yanukovych dismisses the vote as illegal.

Mr Yushchenko agrees to lift a blockade on government buildings but asks supporters to remain on the streets.

30 November: Outgoing President Leonid Kuchma - who backed Mr Yanukovych during the election campaign - says only fresh elections can resolve the stand-off.

29 November: The Supreme Court begins considering allegations of electoral abuses. Mr Yanukovych says he might accept vote re-runs in certain disputed areas.

28 November: Eastern regions threaten to secede if Mr Yushchenko is declared president.

27 November: Parliamentary deputies declare the poll invalid and pass a symbolic, non-binding vote of no-confidence in the electoral commission. Rival protests backing Mr Yanukovych are held in his stronghold of Donetsk.

26 November: Mr Yanukovych and Mr Yushchenko hold talks and agree to seek peaceful solution. Mr Yushchenko demands a re-run of the vote.

Meanwhile, Mr Yushchenko's supporters lay siege to government buildings.

25 November: The Supreme Court suspends publication of the results while it examines the case, after the opposition appeals.

24 November: The official results are published, giving Mr Yanukovych 49.46% and Mr Yushchenko 46.61%.

22 November: The Central Electoral Commission declares Mr Yanukovych the winner. But Mr Yushchenko's supporters reject the result and gather in Kiev amid claims of vote-rigging. In the following days the protests build, despite sub-zero temperatures.

21 November: The second round of voting takes place after an interim period of rising tensions.

31 October: Voting in the presidential election gives Mr Yushchenko a small lead of just 0.5% against Mr Yanukovych and triggers a second-round ballot.

But Western poll observers complain of serious irregularities and say the poll failed to meet international standards.

The vote follows a bitter election campaign marked by allegations of assassination attempts, media bias and intimidation.