Ukrainian Prime Minister Viktor Yanukovych says he will challenge the results of Sunday's election in the country's supreme court.
Official returns, with more than 99 percent of the vote counted, give opposition leader Viktor Yushchenko just over 52 percent of the vote, and Mr. Yanukovych a little more than 44 percent. Voter turnout was put at more than 77 percent.
Europe's top security organization said the presidential election has brought Ukraine much closer to meeting international standards.
The head of an Organization for Security and Cooperation in Europe monitoring group said Monday Ukraine can be proud of taking a great step toward free and fair elections.
Sunday's vote was monitored by about 12,000 international observers, and reports Monday said voting problems and irregularities were much fewer than in the previous runoff that Ukraine's supreme court annulled.
Polish President Aleksander Kwasniewski has congratulated Ukrainian opposition leader Viktor Yushchenko on his victory in Sunday's run-off election.
The Polish leader, who had helped mediate in the political crisis in Ukraine, said Monday the result bodes well for relations between Kiev and Europe. President Kwasniewski invited Mr. Yushchenko to visit Poland next month.
European Commission President Jose Manuel Barroso praised the election results -- and repeated his warning that the divisive campaigning preceding the election should not lead to the breakup of Ukraine.
European Union Commissioner for External Relations Benita Ferrero-Waldner said she plans to visit Ukraine in the near future to prepare for increased cooperation between the country and the EU.