He has called for a second day of protests after tens of thousands of opposition supporters rallied in Kiev over the disputed election result.
The disputed results have heightened fears of a violent standoff between the opposition and the government of the former Soviet state.
The election has sparked intense rivalry between Washington and Moscow, with the US and European Union seeing Ukraine as a buffer to an increasingly authoritarian Russia.
The US and European countries have also cried foul over the result.
But Russian President Vladimir Putin ignored the controversy and congratulated the government candidate Mr Yanukovich for what he said was a “decisive victory”.
Mr Yanukovich assumed the role of president-in-waiting, despite tens of thousands of protesters who massed in Kiev overnight.
“We won. Full stop,” said an angry Mr Yanukovich.
Opposition supporters and the capital’s city council refused to recognise the official results and urged parliament to follow suit.
Two cities in the opposition-dominated west, Lviv and Ivano-Frankivks, also refused to recognise the results, sparking fears that Sunday’s election could further polarise the deeply divided country.
The central security authorities have warned that they are ready to put down any lawlessness “quickly and firmly”.
The opposition said it has evidence of thousands of irregularities, including strong turnouts in government strongholds.
With over 99 percent of polling stations reporting, Prime Minister Yanukovich — who has received endorsement from Russian President Putin — had secured 49.42 per cent of the vote.
Official figures put Western-leaning Mr Yushchenko’s vote at 46.7 percent, but he refused to concede.
“We are launching an organised movement of civil resistance, he told supporters in Kiev.
“The campaign is only just beginning. Don’t leave Independence Square until victory,” he said.
Exit polls earlier showed Mr Yushchenko comfortably led the vote.
Western observers have slammed the vote. The Organisation for Security and Cooperation in Europe said the election failed to “meet a considerable number of OSCE commitments and Council of Europe and other European standards for democratic elections”.
Mr Yushchenko wants to move towards membership of the European Union and the US-led NATO military alliance, while Mr Yanukovich has pledged to make Russian a second official language and allow dual citizenship.