Ukraine has parliamentary elections Sunday. Members of the Organization for Security and Cooperation in Europe arrived in the country last month to launch a voter-awareness campaign.
Council of Europe Secretary-General Thorbjorn Jagland was quoted by Deutsche Welle as saying there were concerns ahead of the election.
"There is a risk that these elections will not be recognized as free and fair," he said.
Jagland said European leaders expected widespread demonstrations in support of jailed opposition leader Yulia Tymoshenko, a former prime minister, ahead of the vote.
Anna Szyptur, a coordinator for the Canadian observer mission in Ukraine, was quoted by the Interfax news agency as saying there was evidence of certain "drawbacks" in the run-up to the elections.
Authorities in Kiev, however, maintained that polling stations were equipped with lock boxes and other devices to prevent tampering. Monitors from all parties taking part in the vote, as well as international groups, will be present at voting stations, officials said.
Tymoshenko was sentenced to seven years in prison after she was convicted on charges she abused her authority in a 2009 gas deal with Russia's Gazprom. Her supporters say the charges were politically motivated.