By Richard Balmforth and Luke Baker
KIEV/BRUSSELS (Reuters) - Ukrainian police fired tear gas at pro-Europe demonstrators and authorities sought to isolate jailed opposition leader Yulia Tymoshenko on Monday as she launched a hunger strike over Kiev's rejection of a European trade pact under pressure from Moscow.
European Union leaders issued unusually strong criticism of Russia, stressing the offer to Ukraine remained on the table despite little indication it would sign the pact with the EU at a summit on Friday as originally planned.
Police clashed with protesters who gathered for a second day in Kiev and speakers urged people to stay on the streets, although numbers were smaller on Sunday, the largest turnout since a pro-democracy "Orange Revolution" nine years ago.
President Viktor Yanukovich, acting to defuse pressure from the streets, which denied him the presidency the first time in 2005, said rejecting the pact had been difficult but unavoidable -- implying EU rules were too tough on the fragile economy.
He pledged to create "a society of European standards".
"My policies on this path always have been, and will continue to be, consistent," he said in a television address which did not mention relations with Russia or refer to EU pressure to release Tymoshenko, his fiercest opponent.
Within minutes of his address, a second round of clashes broke out near Kiev's European Square in which special force units used batons and tear gas for several minutes against a small group of protesters away from the main body of the rally.
Former economy minister Arseny Yatsenyuk, one of the opposition leaders, denounced Yanukovich's address as an attempt "to justify his absurd policy" and Tymoshenko's lawyer told the crowd she would stop eating to persuade Yanukovich to change his mind.
"As a sign of unity with you, I declare an unlimited hunger strike with the demand to Yanukovich to sign the association agreement," declared the 52-year-old Tymoshenko in a message to the protesters read out by her defense lawyer, Serhiy Vlasenko.
The Ukraine prison service said it was stopping all visits to patients in the hospital in the town of Kharkiv where Tymoshenko is being treated, citing a health risk because of an outbreak of respiratory infection in the town.
Public health regulations meant that mass meetings would be suspended too, it said, - something that might rule out any protest demonstrations on behalf of Tymoshenko.
Yevgenia, Tymoshenko's 32-year-old daughter, said that when she went to visit her mother on Monday she was refused entry. "This is the deliberate, unlawful isolation of my mother," she was quoted by the web site of her mother's party as saying. "To take away from a daughter her visit to her mother is humiliating and immoral," she said.
The proposed far-reaching trade and political association agreement with Ukraine was the biggest prize in Brussels' efforts to draw states in the former Communist East closer to the EU fold.
But Kiev suddenly announced last week it had decided instead to seek closer trade relations with Moscow.
The decision followed months of Russian pressure, including threats to cut off Ukraine's gas supplies and impose trade restrictions. Moscow has accused the European Union of putting the squeeze on Kiev, too.
The EU's two most senior officials, European Council President Herman Van Rompuy and European Commission President Jose Manuel Barroso, denounced Russia's actions and said the EU offer remained open.
"The European Union will not force Ukraine, or any other partner, to choose between the European Union or any other regional entity," they said in a joint statement.
"We therefore strongly disapprove of the Russian position and actions in this respect."
Some saw the protests as part of a wider struggle in a country that houses both native Ukrainian and Russian speakers and which many Russians see as culturally part of their nation.
"I have turned out for revolution because I have understood that the promises of Yanukovich to go into Europe were just pure comedy," said Anatoly Gurkalyuk, 33, a builder.
At the end of last week, the EU appeared minded to quietly accept Ukraine's decision to back away from the trade deal. But the protests - with their hallmarks of Ukraine's 'orange' democracy drive of 2004-2005 - look to have spurred the EU into a renewed effort to court Ukraine.
"It is up to Ukraine to freely decide what kind of engagement they seek with the European Union. Ukrainian citizens have shown again these last days that they fully understand and embrace the historic nature of the European association," the joint EU statement said.
While it seems unlikely Yanukovich will change his mind between now and the Vilnius summit, he might still attend the event, which includes a dinner with EU leaders on Thursday night.
He did not say whether he would go to Vilnius in his Monday address and some commentators noted he dropped an habitual reference to 'Eurointegration' in his speech.
EU officials said the occasion might be an opportunity for German Chancellor Angela Merkel, Britain's Prime Minister David Cameron and French President Francois Hollande, to convince him of the benefits of looking West, even if he doesn't budge now.
It remains unclear what Russian President Vladimir Putin said to Yanukovich to convince him to turn away from the EU.
Diplomatic sources in Moscow, Kiev and Brussels have indicated it probably involved a combination of threats to withdraw political support, targeted economic pressure and the inducement of cheaper Russian gas.
Russia set up its own customs union with Belarus and Kazakhstan in 2010 and wants Ukraine, as well as other former Soviet republics, to join it. Ultimately, it sees the customs union as an alternative to the 28-member European Union.
Yanukovich's prime minister reproached the EU for pressing Ukraine to fulfil reform criteria, including releasing Tymoshenko. Mykola Azarov said the IMF's refusal to soften its terms for fresh financial assistance had been 'the last straw'.
EU officials have said Russia told Ukraine that introducing EU rules would have cost as much as $100 billion, while Russia cutting off trade would have hurt the country to the tune $500 billion, although it is not clear over what period.
While an EU free-trade deal might help Ukrainian business and growth over time, it is not a first step towards EU membership, the ultimate prize. And it was not clear whether signing up with the EU would have done much to bolster Yanukovich's reelection hopes in 2015, either.
One of the many issues Brussels wanted Yanukovich to resolve before signing the deal was the imprisonment of former prime minister Tymoshenko, a potential election challenger.
(Additional reporting by Pavel Polityuk and Natalya Zinets in Kiev; Writing by Richard Balmforth and Luke Baker; editing by Philippa Fletcher)