European observers and the United States have criticized the first round of voting on 31 October as falling well short of democratic standards.
Complaints of widespread vote fraud are expected to be discussed today in Ukraine's parliament.
Nearly complete results of the vote show Prime Minister Viktor Yanukovych, an advocate of tighter ties to Russia, with about 40 percent of the vote. Western-leaning opposition leader Viktor Yushchenko has about 39 percent.
The two men are set for a second-round runoff on 21 November.