Several protesters have been injured in the Ukrainian capital, Kiev, after riot police forcibly broke up the remnants of a week-long anti-government demonstration.
The crowd of about 400 protesters, who had been demanding the resignation of President Viktor Yanukovych after he blocked a key European trade deal, were set upon at around 4:30am (0230 GMT) by baton-wielding officers as tear gas filled Kiev's famed Liberty Square.
Journalists were prevented from filming scenes, and a Reuters camera operator was among those injured.
"There is a concern that these protests could grow into something resembling the Orange Revolution," in which Yanukovich was forced from office in 2004, said Al Jazeera's David Chater, reporting from Kiev.
"We are likely to see confrontation rather than compromise," in coming days, he added.
Opposition parliamentarian Oleksandra Kuzhel was quoted by the Interfax news agency on Saturday as saying: "We don't intend to step back." Another protest will be attempted on Sunday, as demonstrators vowed to fight on.
United States officials were among the first diplomats to react to the crackdown.
"The United States condemns the violence against protesters," read a statement from the Kiev embassy. "We urge the government of Ukraine to respect the rights of civil society and the principles of freedom and speech and freedom of assembly."
Rejecting the deal
As many as 10,000 people had gathered in the square on Friday night, after Yanukovich refused to sign a long-anticipated association agreement with EU officials at a summit in Vilnius, Lithuania.
The deal would have moved the former Soviet state closer, politically and economically, to the EU - at the expense of Russia.
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As the deal was being agreed in recent months, Russia punished Ukraine with a series of trade sanctions.
Yanukovych said that his country could not afford to sacrifice trade with Moscow for closer ties to the EU.
A hefty economic aid package to offset Russia's punitive measures could change the situation, however.
"Unfortunately, Ukraine in the last while has turned out to be alone in dealing with serious financial and economic problems," he told European leaders at the Vilnius summit on Friday.
"So today we need from our European partners decisive steps to be taken towards Ukraine to work out, and realise, a programme of financial-economic aid."
Earlier on Friday night, Ukraine's political opposition said that Yanukovich had betrayed hopes of closer integration with Europe.
"Today they stole our dream, our dream of living in a normal country," said Vitaly Klitschko, heavyweight boxing champion and a contender for the 2015 presidential election.
"The failure to sign the agreement of association is treason."
Yanukovich, however, is not without his supporters in the nation of 45 million.
"If we had signed, we would have opened our borders and killed our own manufacturers," Anatoliy Bliznyuk, a parliamentarian from Yanukovich's Regions Party, said.