Ukraine's Orange Revolution parties and their rivals are both claiming victory in early elections, as a vote count nears completion with no clear winner.
Security forces have been ordered to investigate delays in the count
Results based on a count of 97% of the vote give a slim lead to the Orange alliance of Yulia Tymoshenko and President Viktor Yushchenko.
PM Viktor Yanukovych's rival party has the single most votes. It hopes to strike a deal with smaller parties.
Coalition talks are expected to decide which bloc forms the next government.
Just over 60% of the 37.5m eligible voters cast their ballots on Sunday, Ukraine's electoral commission said.
Results based on a count of 97.38% of the vote showed Mr Yanukovych's Party of Regions (PR), which favours closer ties with Moscow, was the strongest single party with 34.15%.
The PR also has the support of the Communists, who secured 5.35% of the vote, giving the alliance a combined total of nearly 40%.
The Yulia Tymoshenko bloc (BYT) polled 30.84% of the vote, while Mr Yushchenko's Our Ukraine-People's Self Defence (NUNS) had 14.30%.
This gave the Orange allies a combined total of just over 45% of the vote.
Alliances agreed with the smaller Lytvyn bloc and the Socialist party - fourth and fifth in the vote count so far - could now decide which group forms the next government.
Currently holding 3.95% of the vote, the centrist Lytvyn bloc of the former parliamentary speaker has yet to indicate which side it wants to ally itself with.
Meanwhile, the Socialists - having polled only 2.91% of the vote - could fail to clear the 3% threshold required to enter the parliament.
On Monday, Mr Yushchenko demanded an urgent investigation into the delays in the counting of votes, particularly in the eastern and southern regions.
In a televised address, the president announced an investigation, warning that those who commit fraud will be punished:
"I am concerned at the delayed vote count in Ukraine's eastern and southern regions, namely in Donetsk, Luhansk and Odessa regions and in the autonomous republic of Crimea," he said.
Earlier, Europe's main election-monitoring body, the Organisation for Security and Co-operation in Europe, said the elections had met international standards.
Battle for influence
Ms Tymoshenko has said she wants to form a new coalition government with President Yushchenko's party - an old ally from the 2004 Orange Revolution.
But Mr Yanukovych has refused to accept defeat in the poll and thousands of supporters of his Regions Party have been gathering in Kiev's main Independence Square.
"This significant support from the Ukrainian people... gives carte blanche to the Party of Regions to form a new, successful government," Mr Yanukovych said on Monday.
The snap election was the third national poll in three years.
It was called in an attempt to resolve a long-running power struggle between West-leaning Mr Yushchenko and Mr Yanukovych, who is viewed as being closer to Russia.
Coalition horse-trading after last year's parliamentary elections took months and plunged Ukraine into the political turmoil which helped trigger the latest snap poll.
The EU, the US and Russia are all vying for influence in Ukraine, which straddles key Russian gas export routes to energy-hungry EU nations.