"We have to conclude that the 2004 presidential election did not meet a considerable number of standards," Bruce George, head of the Organisation for Security and Cooperation in Europe mission, told a news conference. "It was a step backwards from 2002 elections."
Another mission, the European Network of Election Monitoring Organisations (ENEMO), said voting "has been hampered by numerous, serious irregularities".
"ENEMO observers have found repeated instances of voters being turned away because their names didn't appear on voting lists, controlled voting by bussing voters to polls, and serious pressure on students by school administrators," said Edil Baisalov, co-leader of the group.
ENEMO represents civic observers from 16 countries of the former Soviet Union and Eastern Europe, which had monitors at 300 of the 33,000 polling stations set up across Ukraine.
Despite the irregularities, the group said it still expected Viktor Yanukovich, the pro-Russian prime minister, to make it into a second round run-off against opposition leader Viktor Yushchenko on 21 November, as most polls had forecast.
Citing specific incidents, ENEMO said that at least 80 voters were turned away in the first 90 minutes of voting from just one polling station in the capital Kiev, another 60 in Kirovograd, 54 in the town of Vinnitsa, and 20 in the city of Dnepropetrovsk.
"It is unclear why Ukrainian citizens who believed they were on voting lists were not," ENEMO said in a statement.
The group further cited an incident where 850 students were taken by buses to Kiev and instructed to vote for Yanukovich, the official candidate from the party of power who holds strong pro-Russian views.
And in the eastern city of Kharkov, university students were instructed to vote at university facilities, with their names checked off an official list after they had complied.
The group said it would station 1000 observers for the 21 November runoff.
Sunday's vote capped a campaign fraught with intrigue, accusations from both main camps of cheating and one charge of attempted assassination by poisoning, unusual even when compared to the heated polls in other post-Soviet states.
Washington and Europe have tacitly supported Yushchenko's campaign, while Moscow has thrown its weight behind Yanukovich.
Outgoing President Kuchma (L)
and Putin (R) are allies
The US Department of State has threatened to take measures against Ukraine if the vote is found to have been rigged in Yanukovich's favour, while Russian President Vladimir Putin took the unprecedented step of spending three days at the prime minister's side in Kiev last week.
The European Parliament has described the poll as a moment of truth for democracy in the country after a decade of rule under Kuchma, whose record on reform and human rights has been mixed at best.
Analysts have said that a loss for Yanukovich would deliver a blow to the prestige of Putin, who has been trying to see allies secure leadership posts in former Soviet republics.
"The entire Russian government, starting with the president, will look like fools if Yushchenko wins by a large margin," Russian political analyst Leonid Radzikhovsky told Moscow Echo radio.
Although Yushchenko held a slim lead in three of five exit polls published after Sunday's vote, Yanukovich was ahead in the vote count, with more than 80% of ballots counted.
The partial results showed Yanukovich with 41.43% of the popular vote to Yushchenko's 37.92%, after 84.32% of votes had been counted. Turnout was at a record 75%.
Early estimates put Yanukovich
in the lead
Yushchenko's supporters have planned a massive demonstration in support of their candidate for Monday afternoon in central Kiev, with more than half a million people registered to attend.
The initial results made it practically certain that the two Viktors will head into a 21 November showdown, in what could amount to a geopolitical referendum on the future of this nation of 48 million people.
The veteran outgoing president, Leonid Kuchma, will keep hold of the reins of power until then.