Police are storming the main protest camp in Ukraine's capital, Kiev, which has been occupied since November.
Explosions are taking place, fireworks are being thrown and large fires have broken out in Independence Square, known locally as the Maidan.
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.
Meanwhile, Mr Yanukovych's aide said the president was preparing to address the nation, without providing further details.
'Island of freedom'
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'.
They advanced with an armoured vehicle, dismantling barricades and firing stun grenades and water cannon.
Protesters threw fireworks and petrol bombs, and lit fires to .