Police are storming the main protest camp in Ukraine's capital, Kiev, which has been occupied since November.
On Tuesday at least 18 people were killed, including seven policemen, in the worst violence seen in weeks.
Opposition leaders later met President Viktor Yanukovych but failed to find a solution to the crisis.
Vitaly Klitschko, leader of the opposition Udar (Punch) party, told Ukraine's Hromadske TV that the president had given the protesters only one option, leave the Maidan and go home.
Security forces had given protesters a deadline of 18:00 local time (16:00 GMT) to leave Independence Square, the scene of a mostly peaceful protest camp since November.
The city's metro service was completely shut down, and there were reports that cars were being prevented from coming in to the capital.
Then shortly before 18:00 GMT, police announced over loudspeakers that they were about to begin "an anti-terror operation".
Late on Tuesday, the police tried to break through a barricade from the Evropeyska Square, but the attack was repelled.
In a renewed assault shortly after 04:00 local time on Wednesday (02:00 GMT), the police tried to move on the protesters' tents near the main monument on the square.
A number of tents were set ablaze, and the police reportedly again began using water cannon.
In speeches from the main stage, protest leaders urged people already on the Maidan to stand firm, and called on Ukrainians elsewhere to come to the square.
Meanwhile, there are reports of unrest breaking out elsewhere in Ukraine, including the western cities of Lviv, Ivano-Frankivsk.
Mr Yanukovych's aide said the president was preparing to address the nation, without providing further details.
Read more: BBC News - Ukraine crisis: Police storm main Kiev 'Maidan' protest camp
