Yushchenko, in his challenge to the Supreme Court, contends the runoff election was fraudulent.
It is unclear when the court will hand down its decision. The ruling would follow a nonbinding resolution on 27 November by parliament calling the elections flawed.
Yushchenko supporters thronged the area outside the court building in Kyiv today, chanting slogans.
Tension is rising as both sides' positions harden.
"For the first time now, people are talking explicitly not only about the possibility of a bloody civil war, but now in addition there is even the perspective of a separation of parts of the republic from the Ukraine; so that definitely puts the whole developments of the past 48 hours absolutely in a different perspective," political analyst Dick Leurdyk said, noting that the crisis in Ukraine has taken a new and ugly turn.
Yanukovych yesterday visited eastern Ukraine, where he has his support base. At a rally, governors and top officials of some 17 eastern regions accused the opposition of pushing to split the country.
One of the speakers was the chairman of the Luhansk regional legislature, Viktor Tychonov.
"The participants of the [Luhansk regional] legislature decisively condemn the antistate and anti-national actions of the so-called opposition that pushes Ukraine toward a territorial split and catastrophe," Tychonov said. "Therefore, because of the possibility of a 'coup d'etat' and an illegitimate president coming to power, we the participants of the legislature reserve the right to take adequate steps for the self-defense of the citizens of our regions and all Ukraine."
Back in Kyiv, opposition deputy Yuliya Timoshenko has called on Kuchma to dismiss by today Prime Minister Yanukovych and the governors of separatist-minded regions.
Yushchenko sharply criticized the regions yesterday for playing what he called a "dangerous" game with the country.
"This power -- which has completely lost the elections and lost the contest with its own people -- today is trying to play a very politically dangerous card called 'separatism,'" Yushchenko said. "And it is them who, for the first time, put the posters around Kyiv displaying Ukraine divided into three parts, with people of the first, second, and third sort. And now they are trying to develop the idea of creating southeast autonomy."
Analyst Leurdyk, who is a senior researcher with the Netherlands Institute of International Relations, said what is going to happen in the next 48 hours seems "completely open." He said the decision of the Supreme Court will have a key bearing on developments.
"The best outcome, under the present circumstances -- at least it is my guess -- would be that the Supreme Court would say, 'We cannot approve the outcome of the elections, there should be another election,'" Leurdyk said.
Yushchenko's side would accept that, but it's not clear whether Yanukovych's camp would. But Leurdyk said he sees some hope of that.
"Over the weekend, Moscow has said that even they are in favor of a new round of elections, and that makes me hopeful that this will be indeed a realistic option," Leurdyk said.
Yanukovych is broadly seen as pro-Russian. Russian President Vladimir Putin swam against the tide of international opinion when he offered Yanukovych early congratulations on winning the election. The change now in Russia's position could influence Yanukovych.
The United States and the European Union quickly rejected the elections as failing to meet democratic standards.