The opposition alleges Yanukovych supporters massively stuffed ballot boxes. They say the candidate's backers were also issued special absentee-voting documents that allowed them to vote many times at different polling stations.
Yushchenko yesterday addressed the demonstrators, telling them that they were heroes.
"I am proud that in the Ukrainian nation, there are people who can camp out overnight, despite freezing temperatures, not for themselves, but for the sake of the Ukrainian people," Yushchenko said.
Yushchenko called on demonstrators to go to parliament, where the opposition was seeking to pass a motion of "no confidence" in the election.
"I hope that in the parliament, we will find deputies, 226 deputies, who will raise their hands and say that what the Central Election Commission did and what its local branches did, was a betrayal of their oath of office," Yushchenko said.
Yanukovych today went on television to accuse the opposition of being radicals.
"A group of radicals has set itself the aim of splitting Ukraine by violence and illegal methods," Yanukovych said. "They are following a previously prepared scenario and are trying to drag in young people."
The OSCE, the European Union, the United States, and other Western countries have condemned the vote. The EU and U.S. have asked the government to review the result and warned of worsening relations if this is not done.
Russian monitors, on the other hand, mainly support the result. Russian President Vladimir Putin has welcomed Yanukovych's victory.
Putin was an early supporter of Yanukovych and visited Ukraine on the eve of each round of the two-round election to endorse the prime minister.
The deputy head of the European Parliament's committee on relations with Ukraine, Charles Tannock, accused the government of trying to defraud the voters. He addressed crowds in central Kyiv: "I send my greetings on behalf of the people of Europe, the European Parliament of which I am a member, and we salute the courage of the democratic forces of Ukraine. Me and [my colleagues] here were official observers at your election. We came here in the hope that we would see free, fair, and transparent elections. Everything I have seen and heard has been a great disappointment to me in the election itself, but not in the courage and spirit of the people of Ukraine, particularly the young people who are here today."
Four cities in Ukraine, including the western city of Lviv, have recognized Yushchenko as president and said the city and regional administrations have pledged to obey only his orders.