Riot police in the Ukrainian capital, Kiev, have forcefully dispersed hundreds of protesters, beating some with truncheons, witnesses say.
Protest organiser Sergei Milnichenko said tear gas had also been used as police moved in at about 04:30 (02:30 GMT) on Saturday.
It followed fresh rallies against President Viktor Yanukovych's refusal to sign an EU association agreement.
Unconfirmed reports said a number of people had been hurt.
Police said they had decided to clear Independence Square after "a number of incidents", Interfax Ukraine news agency reported.
It was not clear what incidents they were referring to.
More than 1,000 people, most of them students, were in the square when police moved in, activists said.
"It was horrible. We were holding a peaceful demonstration and they attacked us," said protester Lada Tromada.
"They threw us away like garbage."
Witnesses said ambulances were on the scene and some demonstrators were seen bleeding from their heads and arms.
One activist, opposition MP Andriy Shevchenko, tweeted that dozens of people had been hurt and at least 33 taken into police custody.
Reuters news agency said the injured included one of its cameramen and a photographer, who was left bloodied by blows to the head.
The US Ambassador to Kiev, Geoffrey Pyatt, said he was "still working to understand what happened".
He said he "obviously" condemned the violence against peaceful demonstrators, adding: "Will have more to say."
By morning, police had surrounded the square and barely any protesters remained, a BBC correspondent reported.
Several hundred have now gathered at St Michael's Cathedral in the capital, contemplating their next move.
Last week, Mr Yanukovych said he was suspending preparations for signing an EU association agreement that would have opened borders to goods and set the stage for an easing of travel restrictions.
He said pressure from Russia had led him to make his decision. Mr Yanukovych argued that Ukraine could not afford to sacrifice trade with Russia, which opposed the deal.
The agreement was to have been signed on Friday at an EU summit in the Lithuanian capital, Vilnius, and opposition leaders called for a large turnout of protesters to make their feelings known.
On Friday - as Mr Yanukovych attended the EU summit - about 10,000 demonstrators took to Independence Square, carrying Ukrainian and EU flags and chanting "Ukraine is Europe".
There was also a smaller rally involving some 3,000 supporters of Mr Yanukovych a few hundred metres away in European Square.
In Vilnius, EU leaders warned they would not tolerate Russian interference in the bloc's relations with former Soviet republics.
The summit reached provisional accords with Georgia and Moldova.
"The times of limited sovereignty are over in Europe," said European Commission President Jose Manuel Barroso.
EU Council President Herman Van Rompuy said the parties had been "really close" to signing the association agreement in Vilnius, but added: "We need to overcome pressure from abroad."
"We are embarked on a long journey, helping Ukraine to become, as others, what we call now, 'new member states'. But we have to set aside short-term political calculations."
German Chancellor Angela Merkel said the door would always remain open for Ukraine.
Independence Square was the scene of the Orange Revolution in 2004, which saw Mr Yanukovych ousted and a Western-leaning government brought to power.
Mr Yanukovych was elected president five years later, narrowly defeating then-Prime Minister Yulia Tymoshenko, a leading figure of the Orange Revolution.
In 2011 she was sentenced to seven years in jail for abuse of office - a case widely criticised in the West as political revenge.
Ms Tymoshenko has been on hunger strike since Monday over the failure to sign the EU agreement.