Some live rounds have been fired but it is not clear by whom. Protesters are throwing petrol bombs, while police are using water cannon.
Three EU foreign ministers are in Kiev for talks before an EU meeting to discuss possible sanctions.
The health ministry says the death toll in protests this week has risen to 28.
Fires burned at the main protest camp, the Maidan, through the night.
Two armored vehicles have been seen in the street leading towards the square.
Thursday has been declared a day of mourning for the dead.
Most of the victims died during clashes on Tuesday – the bloodiest day since the unrest erupted in late November.
German Foreign Minister Frank-Walter Steinmeier and his counterparts from France and Poland, Laurent Fabius and Radoslaw Sikorski, are expected to meet President Viktor Yanukovych and other government officials on Thursday morning.
The EU ministers will also hold separate talks with the opposition.
Ahead of the Kiev visit, Laurent Fabius called the warring sides to “return to dialogue”, condemning the violence as “unacceptable”.
“Perpetrators of these acts cannot go without sanctions.”
The three ministers will then fly to Brussels for a crisis meeting with EU foreign policy chief Catherine Ashton and other EU foreign ministers.
They will consider whether to impose sanctions, which could include a ban on sales of equipment which can be used for internal repression.
The Ukrainian opposition has been long pressing the EU and US to impose sanctions against senior government officials believed to be responsible for the violence against protesters.
The EU has so far refrained from such a move, preferring to stress dialogue and compromise.
Meanwhile, the US state department announced on Wednesday it had imposed visa bans on 20 members of Ukraine’s government.
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