Western leaders have condemned the jailing of former Ukraine Prime Minister Yulia Tymoshenko.
The US and EU said the trial was politically motivated and Russia's foreign ministry said the ruling had a "clear anti-Russian subtext".
Tymoshenko was jailed for seven years for acting beyond her powers in sealing a gas deal with Russia in 2009.
Russian Prime Minister Vladimir Putin, who signed the deal, said he could not understand the court's verdict.
"It is dangerous and counterproductive to cast the entire package of agreements into doubt," said Mr Putin.
The judge ruled that Tymoshenko had "used her powers for criminal ends" by pushing Ukraine's state energy firm Naftogaz into a deal with Russia's Gazprom.
The US called for the release of Tymoshenko and decried the "politically motivated prosecution".
"The charges against Mrs Tymoshenko and the conduct of her trial... have raised serious concerns about the government of Ukraine's commitment to democracy and rule of law," the White House said in a statement.
Several European countries were scathing about the sentencing, Sweden's Foreign Minister Carl Bildt labelling it a "political show-trial".
Rights watchdog the Council of Europe accused Ukraine of "criminalisation of political decision-taking", while Amnesty International demanded that Tymoshenko be set free.
EU foreign policy chief Baroness Ashton, who has been involved in negotiating a free-trade deal with Ukraine President Viktor Yanukovych, said the trial "did not respect international standards".
"This unfortunately confirms that justice is being applied selectively in politically motivated prosecutions of the leaders of the opposition and members of the former government," she said.
Analysts say the jailing of Tymoshenko has threatened to torpedo the free-trade deal, which was due to begin in December.
Tymoshenko's supporters say Mr Yanukovych orchestrated the trial to get rid of his bitter political rival.
The president had been under pressure from Western governments and rights groups to downgrade the charges against Tymoshenko to administrative offences rather than criminal.
In comments after the sentencing, Mr Yanukovych said he understood the reaction to the "regrettable" case, adding that Tuesday's verdict was not final.
"There is the court of appeal ahead and what decision it will take and under which legislation has great importance," he said.
Hundreds of Tymoshenko supporters gathered outside the court to wait for Judge Rodion Kireyev's verdict, some getting into scuffles with security forces.
He jailed her for seven years and banned her from political office for three years.
He also ordered her to pay back 1.5bn hrivnas ($186m; £119m) he said had been lost by Naftogaz as a result of the deal she signed.
Allies no more
Russia pipes gas to Western Europe across Ukrainian territory and relations between the two ex-Soviet states have long been dogged by disputes over transit fees and unpaid bills.
As the verdict was read out over several hours, Tymoshenko stared at her iPad, apparently not listening to the judge.
She has been in custody for contempt of court since 5 August.
Tymoshenko was the heroine of the Western-leaning Orange Revolution - the sudden street protests that erupted after a fraudulent presidential election in 2004.
She was made prime minister shortly afterwards.
But the next few years saw the reform movement stagnate.
Tymoshenko fell out with her Orange allies, and their constant bickering paralysed the country just as it was facing a deep economic crisis.
In 2010, Tymoshenko was forced into opposition as Mr Yanukovych rose to power.
Some of her former Orange allies - including former President Viktor Yushchenko - testified against her in the court case.