The liberal leader claimed foul play after several exit polls put him ahead of rival candidate and Prime Minister Viktor Yanukovich on Monday.
"I believe in my victory but the government ... has staged total fraud in the elections in the Donetsk and Lugansk regions," Yushchenko said.
"I do not trust the central electoral commission," he added, calling for a rally in the centre of Kiev on Monday morning.
Exit poll suspicions
The Kremlin's favourite, Yanukovich, was ahead in Ukraine's presidential election with 50.27% of the vote after a third of ballots were counted, the central electoral commission said. Yushchenko received 46.18% of the ballot at the same stage.
But exit polls had showed Yushchenko winning the election, by between three and 11 percentage points, although Yanukovich's campaign immediately dismissed the data as "inaccurate and unscientific", saying the prime minister was the victor.
The discrepancy between the exit polls and the official results heightened concerns of a violent stand-off between the opposition and the government of this former Soviet state.
Dr Marko Bojcun, director of the Ukraine Centre at the Metropolitan University in London, told Aljazeera.net on Monday the election was one of the most important in Eastern Europe in 15 years.
Concluding that the result will determine whether about 50 million people turn towards Europe or the Russian Federation for political support, Bojcun described the options left to Yushchenko:
"I don't foresee a situation like Georgia or Yugoslavia, but by calling his supporters into the streets we are seeing Yushchenko applying pressure on the electoral commission to produce a result quickly and limiting the possibility of any falsification," Bojcun said.
Yushchenko says observers
were prevented from monitoring
The analyst broadly agreed that Prime Minister Yanukovich had been correctly labelled as a more authoritarian, soviet-style leader - unlike his much more transparent rival.
"It is fair to compare Yanukovich to the leaders of countries such as Belarus and Kazakhstan."
Support for demonstration
And another opposition leader Yulia Timoshenko called on "Ukrainians to put their work and occupation aside to do their duty as a citizen and not allow the authorities to steal this election".
Yushchenko pledged that any demonstration "will be peaceful and in line with Ukrainian laws" with the current Kuchma regime vowing to maintain public order.
The opposition notably accuses the government of inflating the turnout in Donetsk, an eastern coal-mining area where the prime minister hails from, which reached 96% according to the electoral commission.
The nationwide turnout was 79%.
The nationalist Ukrainian-speaking west and centre supports 50-year-old Yushchenko, while the industrialised Russian-speaking east backs 54-year-old Yanukovich.
Journalists and opposition observers were expelled from numerous polling stations in two eastern Russian-speaking regions, contender Yushchenko said.
Foreign observers reported many irregularities in Sunday's election. US President George Bush served notice to President Kuchma that the United States would review its relations with Ukraine if the presidential vote was not fair.