Exit polls suggested Poroshenko, 48, a billionaire chocolate magnate with business interests in Ukraine that include television stations and manufacturing facilities, received 55 percent of votes cast. If the predictions are maintained, there will be no need for a runoff election, required only if the leading candidate receives fewer than 50 percent of the total vote.
No voting booths were open in the eastern Ukraine city of Donetsk, and only seven of 12 district electoral commissions in the Donetsk region, largely under control of pro-Russian separatists, were in operation. Throughout Ukraine, there were no reports of violence or difficulties with the election process.
The special election, Saturday and Sunday, was called to replace Viktor Yanukovych, who was deposed as president in February. His departure was followed by Russia's invasion and annexation of Crimea, and violence in eastern Ukraine between pro-Russian separatists and Ukrainian military and law enforcement officials.
Poroshenko declared victory Sunday night at a victory celebration in a Kiev arts center. The event was attended by former boxing champion Vitali Klitschko, a leader of the protest movement that deposed Yanukovych. Sunday Klitschko was elected mayor of Kiev.
"These elections determine the future of our country," Poroshenko said in thanking Ukrainians for turning out to vote. "We will do the absolutely unique transformation of our country, with zero tolerance (of) corruption, a very good investment climate, and independent court system, all the necessary things to attract business."
Poroshenko's platform had a decidedly pro- Western tilt and has pledged to sign the trade agreements with Europe that Yanokovych chose not to, provoking the uprising, but some Ukrainians remain skeptical of Poroshenko's motives. Although he has promised an end to corruption, he is a billionaire and a veteran of Ukrainian parliamentary and administrative politics. He also has significant business interests in Russia.
Saturday Russian president Vladimir Putin agreed to" respect any choice made by the Ukrainian people," and the vote Sunday had the support of western leaders, who view it as a step to installing a pro-western democracy in Ukraine.