Supporters of former opposition leader Yulia Tymoshenko confront the police at a protest in Kiev on Monday.
A summit intended to bring Ukraine into Europe's mainstream foundered on Monday after the European Union said it would not sign a landmark political and trade deal until Kiev resolves the case of jailed opposition leader Yulia Tymoshenko.
Negotiations were finished on the agreement, which would create a free-trade zone and establish deeper ties, but European Council President Herman Van Rompuy said signing and ratifying it "will depend on political circumstances."
"Our strong concern is primarily related to the risks of politically motivated justice in Ukraine. The Tymoshenko trial is the most striking example," he told President Viktor Yanukovich.
The summit, four years in preparation, had been intended to mark the start of a new strategic relationship between the EU and the former Soviet republic, which has made European integration a priority while managing strong ties with Russia.
But during two hours of talks, Mr. Van Rompuy and European Commission President Jose Manuel Barroso appeared to have made little headway in persuading Mr. Yanukovich to relent and bring about the release of Ms. Tymoshenko and other opponents.
Ms. Tymoshenko was last seen publicly last week, looking pale and gaunt, on a video clip filmed of her in a prison bed by the authorities, apparently against her will.
The 51-year-old politician herself had made a plea from her cell for the association agreement to be signed – despite her plight – for the good of Ukraine.
Despite the setback, Mr. Yanukovich said Ukraine still sees its future in the European Union at some point.
With 45 million people, Ukraine is the most populous former Soviet state apart from Russia itself, and building a closer relationship is an important strategic goal for Brussels. The stalling of the pact is a setback for some Ukrainian businesses, which covet access to Western Europe's markets and investment.
Ukraine is also the main transit route for Russian natural gas into the EU, which relies on Moscow's energy resources. Mr. Yanukovich said strengthening Europe's energy security remained an important part of Ukraine's co-operation with the EU and it would continue to modernize its gas transport system with the support of the EU and its financial institutions.
But efforts to bring Kiev closer to the European mainstream have been stalled since the sentencing of Ms. Tymoshenko in October.
An implacable foe of Mr. Yanukovich, she was sentenced to seven years in prison for abuse of office while she was prime minister. She called her trial a "lynching" by her adversary.
The EU says her trial raises questions over the democratic credentials of Mr. Yanukovich's leadership and his commitment to the bloc's fundamental values.
Any deal to bring Ukraine into the EU would still need to be ratified by the parliaments of all the 27 EU states and the European Parliament before it could be implemented, a process that at best could take many months.
The bloc is itself split over Ukraine, with countries such as Poland, Ukraine's immediate neighbour and the outgoing EU president, keen to seize the opportunity to pry Ukraine away from Russia's grip, while other EU members refuse to give ground on the principle of democratic values.
Though Mr. Yanukovich put a brave face on the outcome, a failed summit leaves him and his government with a weakened hand in dealings with Moscow, from whom it is seeking a more advantageous gas-pricing deal than one negotiated by Ms. Tymoshenko.