The ruling Party of Regions, led by Ukrainian President Viktor Yanukovych, took more than 35 percent of the vote in the elections. An opposition slate led by jailed former Prime Minister Yulia Tymoshenko took second place with about 20 percent.
The observer mission from the Organization for Security and Cooperation in Europe said the "democratic progress appears to have reversed in Ukraine."
British Minister for Europe David Lidington said the elections, seen as influenced heavily by economics, lacked balance and transparency.
"We regret that Ukraine wasted an opportunity to show firm and consistent commitment to democratic principles," he said in a statement.
Tymoshenko announced Monday she was staging a hunger strike to protest the elections.
EU foreign policy chief Catherine Ashton and Enlargement Commissioner Stefan Fule expressed disappointment that Kiev's justice system prevented leaders such as Tymoshenko from competing.
"We reiterate our regret that the consequences of trials that did not respect international standards have prevented opposition representatives from standing in the parliamentary elections and call on the authorities to address this matter and take further steps to reform the judiciary to avoid their recurrence," they said in a statement.