Opposition and West-leaning candidate Viktor Yushchenko, who called hundreds of thousands of protesters into the streets, told parliament on Tuesday that Ukraine "is on the brink of civil conflict".
He accused the outgoing president and Prime Minister Viktor Yanukovich of responsibility for electoral fraud which produced the results that have Yanukovich poised to be named president.
Supporters prompted Yushchenko to read the oath of office after the conclusion of a tumultuous session of parliament that was boycotted by Yanukovich allies.
He read the oath with his hand on a Bible, opened a window in the parliament building and addressed a sea of supporters outside who have turned out for a second day of raucous protest, which has the centre of the capital seething with anger.
Yushchenko read the oath of
office in parliament
Speaker after speaker stepped up to the microphone in Kiev's Independence Square to defiantly pledge loyalty to Yushchenko.
"We are fighting for democracy and we will win," declared Ihor Ostash, an opposition parliamentary deputy, draped like others in the orange campaign colours of Yushchenko's camp.
The parliamentary session ended without taking any decision on the aftermath of the poll.
"We are sliding towards the abyss. It is amoral and criminal to pretend nothing is happening in the country," parliamentary speaker Volodymyr Lytvyn told deputies at the debate's start.
With passions running high against the background of a warning on Monday from security forces that they would crush unrest quickly and firmly, the political split could turn violent.
Yanukovich, who has been congratulated by his most powerful backer, Russian President Vladimir Putin, has not declared victory. But he virtually assumed the mantle of president on Monday by appearing on television beside the national flag to denounce Yushchenko and his supporters.
"We are sliding towards the abyss. It is amoral and criminal to pretend nothing is happening in the country"
Ukrainian parliamentary speaker
Banners in the sea of protesters included the Georgian red and white flag - a reminder that 23 November was the first anniversary of Georgia's mass "rose revolution" that toppled veteran leader Eduard Shevardnadze and elected a pro-Westerner.
But it was uncertain if Ukraine's liberals could force similar changes.
Protesters, breaking off from sipping soup from vacuum flasks, burst into sporadic chants of "Yushchenko, Yushchenko".
The election gave Ukrainians a stark choice. Yanukovich sees closer ties with Russia as the key to prosperity, while his rival calls for gradual integration with the West.
It also underlined the divide between the nationalist west and the industrial Russian-speaking east that backed Yanukovich.