Yushchenko's main rival, Prime Minister Viktor Yanukovych, was leading in yesterday's run-off presidential election, despite exit polls showing Yushchenko ahead.
Latest official returns show that with more than 99 percent of ballots counted, Yanukovych won over 49.7 percent of the votes, with Yushchenko taking 46.7 percent, Ukraine's independent Channel 5 reported.
An exit poll released immediately after polling stations closed put Yushchenko in the lead, with 54 percent to 43 percent.
Addressing demonstrators today, Yushchenko accused the authorities of fraud: "[The authorities] will work in their usual way and try at any cost to create the deepest possible falsification. If this falsification does not help, they will use forceful methods, including canceling the election."
Yushchenko, who yesterday accused the Ukrainian authorities of carrying out a "coup," urged supporters to continue their protests "until victory."
Some in Ukraine believe street protests could eventually force the government to concede defeat and leave power, like what happened last year in Georgia after disputed parliamentary polls.
But Ukraine's outgoing President Leonid Kuchma has warned his opponents they should not expect a "revolution."
Ukrainian opposition media today accused the government of bringing antiriot police and paramilitary troop reinforcements to Kyiv to deal with any possible unrest.
No obvious police presence was reported near Independence Square on today. But law enforcement officers could be seen near the barbed-wire-protected building which houses the Central Election Commission (CEC) in Kyiv's central Pechersk district. The German Press Agency (dpa) reported seeing armored personal carriers and Interior Ministry troops armed with water cannon.
Yushchenko has accused the CEC of deliberately delaying the vote counting operations in order to manipulate the results.
Addressing reporters today, CEC chairman Serhiy Kivalov denied the count was slanted in favor of Yanukovych: "I would like to remind everyone the famous adage that says 'make haste slowly.' The vote-counting process in itself is interesting. However the outcome of the vote is of no less interest. We're doing everything possible so that we get official and reliable results as quickly as possible."
Authorities in neighboring Russia support Yanukovych. But some Russian opposition parties have openly declared their support for Yushchenko.
Boris Nemtsov, the co-chairman of Russia's Union of Right Forces and a former deputy prime minister, addressed the crowd on Kyiv's Independence Square: "I support Yushchenko, not only because he is my friend and because we worked very well [together] when I was in the Russian government and he was running the Ukrainian one. I support him because he is an honest and upright man."
But another Russian politician, State Duma parliamentarian and CIS Affairs Committee member Konstantin Zatulin, said he believed Yanukovych had clearly won. He said yesterday: "I would like to tell the Ukrainian opposition that not all governments are ready to leave through the back door as it happened in Georgia."