Riot police encircled central Kiev on Monday night, raising fears the authorities were preparing a crackdown on anti-government protesters just hours after Ukraine's president said he was prepared to hold talks with opposition leaders.
The ominous police build-up came as the EU's foreign policy chief Catherine Ashton was preparing to fly into Kiev on Tuesday for talks with president Viktor Yanukovich.
Vitali Klitschko the heavyweight boxer-turned-opposition leader, warned that any move to clear protesters from the streets would be a sign that Mr Yanukovich was setting up a "police state".
Hundreds of riot police appeared to have surrounded the whole of the city centre, though they remained a short distance away from Independence Square, epicentre of the protests where thousands of demonstrators have camped out.
But three metro stations around the square were closed, and shopping malls above and below the square closed down operations early for the first time since protests began 19 days ago.
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Elsewhere in the city, masked men with machine guns late on Monday raided the headquarters of the Fatherland party of jailed opposition leader Yulia Tymoshenko, taking away computer servers, with one person present calling the scene "insane". Some opposition party and independent newspaper websites were also blocked.
Busloads of riot police were beginning to tear down barricades set up by protesters near government buildings in the capital, and were reported to have surrounded a group of demonstrators outside the presidential administration building.
In a possible precursor to action by law-enforcement authorities, Ukraine's prosecutor-general Viktor Pshonka warned in a statement that the protests had turned into "blatant aggression" and were preventing government bodies from operating normally.
"Do not test the patience of government. Do not provoke law enforcement. Lift the blockades of premises and of transport," the statement said.
US vice-president Joe Biden had expressed "deep concern" about the "growing potential for violence" in Ukraine in a phone call to Mr Yanukovich, the White House said on Monday night. Mr Biden called on Ukraine's president to "de-escalate the situation" and begin a dialogue with opposition leaders.
The moves came after hundreds of thousands of Ukrainians flocked to Kievon Sunday in the biggest demonstration since the current protests began – and the largest since Ukraine's 2004 pro-democracy "Orange" revolution.
Protesters are angry over Mr Yanukovich's decision to back away from signing a landmark integration deal with the EU in favour of stronger ties with Russia, and over the brutal dispersal of a small tented protest nine days ago.
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As night fell on Monday, Mr Klitschko told the Financial Times he planned to stay on the streets through the night to ensure protesters' safety.
"If Yanukovich storms the protesters tonight, on the eve of [Lady] Ashton's visit, then it is a sign that he has chosen to turn the country into a police state," he warned.
Standing by protesters' barricades outside Ukraine's interior ministry, Mr Klitschko said he was prepared to hold talks with Mr Yanukovich – but only if the government released several demonstrators arrested in the past week.
In the first hint of any potential compromise, Ukraine's president earlier said he backed a proposal by Leonid Kravchuk, the country's first post-Soviet president, for an "all-national round table" with government and opposition representatives.
But his comments appeared at odds with events unfolding on Kiev's streets, with authorities seeming intent on clearing protesters from at least some parts of the city.
Analysts warned that with military veterans, including former members of Soviet occupying forces in Afghanistan, among the protesters, any attempt at a clampdown could spark widespread bloodshed.
It could also leave Mr Yanukovich ostracised by the international community, and utterly reliant on Russia.
Ukraine's president turned his back on a landmark integration deal with the EU nearly three weeks ago in favour of strengthening ties with Moscow – sparking the protests. But people who know him say Mr Yanukovich fears Vladimir Putin and is wary of being left with the Russian president as his only ally.
Earlier on Monday, José Manuel Barroso, European Commission president, reiterated his warning to Mr Yanukovich to "show restraint" and "not use force against people that are demonstrating peacefully".
Mr Barroso appeared to put himself on the side of the anti-Yanukovich protesters, saying they were "fighting for Ukraine and for their future" and comparing them to earlier advocates of EU membership in Poland and the Baltic states.
"When we see in the cold streets of Kiev men and women with the European flag, fighting for that European flag, it is because they are also fighting for Ukraine and for their future," Mr Barroso said.