Petro Poroshenko, who looks set to win Ukraine's presidential election, says he wants to "end war and bring peace".
Official results are expected on Monday but exit polls suggest the billionaire confectionary magnate has won the election with about 56% of the vote.
He said his first step as president would be to visit the eastern Donbass region where pro-Russian separatists have seized control in many areas.
He also said Kiev would never recognise Russia's "occupation" of Crimea.
The election came three months after pro-Moscow President Viktor Yanukovych fled Kiev amid bloody street protests and calls for closer ties with the EU.
Since then, Russia has annexed the Crimean peninsula in southern Ukraine and armed separatists in the eastern Donbass provinces of Luhansk and Donetsk have declared independence from Kiev.
Ukraine's interim government is engaged in an offensive in the east to quash the uprising that has left dozens dead.
Pro-Russian separatists severely disrupted voting there. No polling stations were open in Donetsk city, and across the region only seven out of 12 district electoral commissions were operating.
However, the central elections commission said about 60% of Ukraine's 35.5 million eligible voters turned out.
"My first decisive step will be aimed at ending the war, ending chaos, and bringing peace to a united and free Ukraine," Mr Poroshenko said at a press conference in Kiev.
"I am certain that our decisive actions will bring fairly quick results."
He also promised a dialogue with people in eastern Ukraine if he is elected.
"For those people who don't take (up) weapons, we are always ready for negotiations to guarantee them security, to guarantee their rights, including speaking the language they want," he said in English.
Mr Poroshenko said he would also like to negotiate a new security treaty with Moscow.
Although he strongly backs closer ties with the EU, Mr Poroshenko also stresses the need to normalise ties with Russia.
Russian President Vladimir Putin has promised to recognise the result of the election.
Kiev and the West accuse Russia of fomenting separatist sentiment - a claim President Putin denies.
US President Barack Obama hailed the election as an "important step forward in the efforts of the Ukrainian government to unify the country".
UK Foreign Secretary William Hague called the election a "decisive signal" of Ukraine's support for reform.
If the exit polls are confirmed, there will be no need for a run-off vote next month.
Former Prime Minister Yulia Tymoshenko was Mr Poroshenko's nearest rival with 12.9% of the vote, according to the exit polls.
After polls closed, Mr Poroshenko appeared on stage beside former boxing champion Vitali Klitschko, who had once been tipped as a presidential candidate but later decided to support Mr Poroshenko.
In local elections also held on Sunday, Mr Klitschko looked set to become the mayor of Kiev.