The first round of Ukraine's presidential election on 31 October failed to meet international standards, Western poll observers said.
Observers criticised numerous election shortcomings
"We have to conclude that this election did not meet a considerable number of ... European standards for democratic elections," an OSCE statement said.
Prime Minister Viktor Yanukovych will face a run-off with his rival Viktor Yushchenko, as they are in a dead heat with nearly all votes counted.
Mr Yushchenko is seen as more reformist and pro-Western than Mr Yanukovych.
With nearly 95% of the votes counted, Mr Yanukovych was only marginally ahead of Mr Yushchenko - with 40% against 39%.
The Organisation for Security and Cooperation in Europe (OSCE) said the election was "a step backwards from the 2002 (parliamentary) elections".
The election is to be decided by a run-off on 21 November.
Mr Yanukovych, backed by outgoing President Leonid Kuchma, fought a bitter election campaign against Mr Yushchenko, marked by claims of dirty tricks and media bias.
Ukraine's 37 million voters were asked to choose between 20 candidates contesting the election.
Yanukovych, right, is backed by both Kuchma and Putin
The OSCE mission included some 600 observers.
According to the OSCE's preliminary statement, "the very high participation of the electorate and civil society in the election process show encouraging signs for the evolution of Ukrainian democracy".
But it listed several concerns about the electoral process, including:
- Bias by the state media
- Interference by the state administration in favour of Prime Minister Yanukovych
- Disruption or obstruction of opposition campaign events by the state authorities
- Inadequacies in the Central Election Commission's handling of complaints.
A separate group of observers reported on Sunday that there had been significant irregularities, with names missing from electoral rolls and voters being turned away from polling stations.
The OSCE mission chief, Geert-Hinrich Ahrens, said his team of observers appreciated the Ukrainian authorities' co-operation, but "we regret that they did not create equal campaign conditions".
"The election also came up short on other counts, such as the failure to allow Ukrainian civil society to directly observe the process."
In an interview with Ukrainian TV, Central Electoral Commission head Sergei Kivalov acknowledged "the biggest problem has been the electoral register, everybody is talking about it".
He said citizens had lodged complaints, and promised to "analyse each situation" after the vote. He said those responsible for the registration problems would be called to account.