The polls will close at 10 p.m. (7 p.m. GMT) for the ex-Soviet state's 37.5 million registered voters. Turnout must be at least 50% for the elections to be valid. Election officials plan to announce preliminary results Monday.
President Viktor Yushchenko said Sunday he would recognize any outcome of the vote and would not allow fresh rallies to overturn results.
"I will acknowledge the election results whatever they may be. There will be no more Maidan," Yushchenko said, referring to the 2004 "orange revolution" protests on the central Maidan Square, which led to his victory over Viktor Yanukovych in a presidential rerun.
Opinion polls put Prime Minister Yanukovych's Party of Regions, supported by the Russian-speaking east and south of the country, in the lead. Yushchenko hopes, however, for a post-election alliance with his "orange" ally and former prime minister, Yulia Tymoshenko.
Tymoshenko's eponymous party and the president's Our Ukraine-People's Self Defense bloc are expected to come second and third respectively. By joining forces they could gain control of the Supreme Rada and form a coalition government. But it is not clear whether their plans will survive the election outcome and subsequent coalition talks.
President Yushchenko, 53, who was the first of the political leaders to cast his ballot Sunday, said he hoped the elections would bring stability to the country and end political rows.
"[The elections] should put an end to the conflict that broke out in parliament eight months ago. This is what the Ukrainian people expect from the political parties running in today's election," he said.
Echoing the president's statements, Yanukovych, 57, who returned to the political scene as premier after his party won a majority in parliament last March, said the country would face no new elections any time soon, adding he was positive the Party of Regions would win.
"I am positive we will win the elections, and a coalition of pragmatic political forces will be formed," Yanukovych said, without elaborating.
The Communists, Yanukovych's allies in the parliamentary coalition, are the only other party likely to make it into the 450-seat Supreme Rada, enjoying 3.3% of support, according to opinion polls. No other party among the 20 listed on the ballot is likely to overcome the required 3% threshold.
The parliamentary speaker and Socialist Party leader said Sunday he expected massive vote rigging.
"I hope our activists will not allow any fraud, but I know preparations have been made to falsify votes," Oleksandr Moroz said.
Ukraine's top election official, Volodymyr Shapoval, said Sunday that voting was proceeding without major violations. He said over 3,000 foreign experts were monitoring the elections.
Earlier this week, a number of parties threatened mass rallies and new court battles to protest against possible fraud, with Yanukovych's supporters setting up a tent camp on Maidan Square, saying they would not leave until their victory was announced.
Yanukovych, accused of a power grab, fiercely opposed the president's move in April to disband parliament and call new elections. The two foes agreed on the September 30 vote following months of litigation and street rallies reminiscent of the "orange revolution."