Ukrainian world boxing champion and popular opposition leader Vitali Klitschko announced on Thursday he would challenge President Viktor Yanukovych in elections due in March 2015.

But his soaring political ambitions were dealt an immediate blow by the adoption of a legal amendment barring people who spend much of their time living or working abroad from running for president.

Klitschko -- a polyglot who reads Fyodor Dostoyevsky novels between gruelling training sessions -- said he was becoming a candidate so long before the vote to bring clarity to the country's bruising and confusing politics.

"In order to put an end to various rumours and attempts to finish me off as a potential candidate, I would like to announce the following: I am running for president," he told a session of parliament.

The 42-year-old heavyweight champion is an ally of jailed opposition leader Yulia Tymoshenko and one of the most popular figures in the former Soviet republic of 45 million people.

His anti-corruption UDAR (Punch) party rose out of nowhere to claim third place in parliamentary elections last year and Klitschko himself has led recent public opinion polls.

Klitschko's presidential ambitions have been rumoured for months but the timing of the announcement still came as a surprise.

The European Union is now putting intense pressure on Ukraine to release and pardon Tymoshenko -- a former prime minister jailed on disputed abuse of power charges -- and Klitschko had previously said he might bow out of the race in favour of his political ally.

Tymoshenko's release and pardon would clear the way for Ukraine to sign a vital association agreement with the EU, the first step toward membership in the 28-nation bloc -- a deal that Yanukovych has sought.

It remains unclear whether the Ukrainian leader will indeed pardon Tymoshenko and set the stage for her own presidential run.

But the prospects have introduced new complexities to Ukrainian politics and prompted a Tymoshenko ally to introduce a new measure that passed parliament Thursday barring those with the right of residency in another country from running for president.

'Viable political rival'

Klitschko spent much of his time in previous years living and training in Germany -- a country whose language he speaks and where he won many of his boxing titles.

"Yes, I have residency rights in Germany," he told the parliament session. "But this does not contravene Ukrainian law or annul my citizenship."

Analysts said Klitschko had no choice but to announce his candidacy because of the new residency rule -- a measure that may still be altered or challenged in parliament.

"His announcement was forced by today's events," said Volodymyr Fesenko of the Penta political research institute.

The analyst added that Klitschko's chances at becoming the next president were "not bad" despite the current efforts to derail his campaign.

"We would not be seeing these (political) games if Yanukovych did not view him as a viable political rival," he said.

Fesenko pointed to recent polls showing the boxer beating Yanukovych -- who is expected to announce plans to run for another term later this year -- in a second-round runoff.

Klitschko and his boxing brother Wladimir -- who between them currently hold all the major versions of the world heavyweight title -- are celebrities in Ukraine whose vivid personalities and rags-to-riches story clashes with the dour images of many in the Ukrainian ruling elite.

Klitschko's relatives suffered along with millions of others during the famines that befell Ukraine and some other regions of the Soviet Union in 1932-1933 during Stalin's tyrannical rule.

Klitschko himself was born into a military family then serving in the ex-Soviet republic of Kyrgyzstan. His family also lived a part of the 1980s in the former Czechoslovakia.

The brothers quickly turned to sports for recreation. Klitschko initially practised kickboxing before finding his calling as a boxer in 1995.

Klitschko formed his pro-European UDAR party in 2010 after finding himself being pressed by the Ukrainian media to add his voice to the raging debate over Tymoshenko's fate.