| Opposition presidential candidate Viktor Yushchenko at a rally in Kiev. (Reuters)
Kiev, Nov. 23 (Reuters): Tens of thousands of Ukrainian Opposition supporters marched on the president's office today seeking to install their leader in office, defying poll results they said had been rigged by a pro-Moscow government.
Hours earlier, pro-western Opposition head Viktor Yushchenko symbolically took the oath of office while some 200,000 backers rallied outside parliament in driving snow and freezing temperatures.
As the crowd moved in a sea of orange Opposition flags and banners towards the presidential offices, formally occupied by outgoing President Leonid Kuchma, witnesses said anti-riot police were taking up positions near the offices.
Demonstrators later stopped their march and faced police equipped with riot shields and protective headgear.
Yushchenko has accused authorities of staging mass fraud to deny him victory in Sunday's run-off which the establishment candidate, Prime Minister Viktor Yanukovich, appeared to have won on the basis of official returns.
Kuchma, who endorsed his prime minister in the election, broke two days of silence to urge both candidates to talk. He warned western countries that their criticism of election procedures could stoke tension in Ukraine. Russian President Vladimir Putin, a supporter of Yanukovich, called for a non-violent solution of the crisis within the framework of the law.
Yushchenko's dramatic gesture declaring himself president came after parliament's speaker hurriedly closed a tumultuous session to ensure that no constitutional rules were violated. No other decision was taken on the disputed election outcome.
Placing his hand on a Bible as his allies in the chamber sang the national anthem, he read the oath, opened a window in the building and spoke to supporters outside.
Yushchenko later reappeared before crowds massed on vast Independence Square, urging them to disregard the cold, remain overnight and guard a tent encampment on Kiev's main street.
'What we started today must be defended every day,' he said. 'We urge you to come to the square in the same numbers. Tomorrow you will be especially needed for the country. One of his lieutenants, Yulia Tymoshenko, urged supporters to proceed to Kuchma's nearby headquarters.
'We are going to go to the presidential administration in a peaceful way, without breaking anything,' Tymoshenko, head of a faction in parliament, said as Yushchenko looked on. 'And either they will give up their power, or we will take it.'
Yushchenko had told parliament that Ukraine, convulsed by two days of protests backing him in Kiev and in nationalist western Ukraine,'is on the brink of civil conflict'.
At a session boycotted by the prime minister's allies, he accused Kuchma and Yanukovich of direct responsibility for the electoral fraud. Liberals had stood up one after the other in the chamber and urged Yushchenko to proceed with the oath. Speaker Volodymyr Lytvyn had opened the debate by warning that Ukraine was 'sliding towards the abyss.