Ukrainian police are storming the main anti-government protest camp in the capital, Kiev, after months of occupation.
Explosions are taking place, fireworks are being thrown and large fires have broken out in Independence Square, known locally as the Maidan.
Police have deployed water cannon.
During the day, at least nine people were killed, including two policemen, as protesters and security forces clashed in the worst violence in weeks.
Security forces had given protesters a deadline of 18:00 local time (16:00 GMT) to end the unrest and had surrounded Independence Square, which has been the scene of a mostly peaceful protest camp since November.
The city's metro service was suspended, and there were reports that cars were being prevented from coming in to the capital.
Protest leaders had insisted they would not retreat, though but warned that they could not "exclude the possibility" of the authorities using force.
Shortly before 18:00 GMT, police announced over loudspeakers that they were about to begin "an anti-terror operation".
Then they began advancing with an armoured vehicle. Police dismantled barricades and fired stun grenades and water cannon.
Protesters lit fires and threw fireworks. Large fires are burning in the square and have destroyed many tents.
Despite the clashes, large numbers of people are still in the square. Protest leaders are on the main stage, encouraging people to stay.
"We will not go anywhere from here," said opposition leader Vitaly Klitschko.
"This is an island of freedom and we will defend it."
Mr Yatsenyuk, who leads the Fatherland party, appealed to President Viktor Yanukovych to "call off your chain dogs who are shooting at civilians".
"There are people here, children, youth and women. We did not start this confrontation. We are urging you to pull the law enforcement bodies back 200 metres. Stop the bloodshed and call a truce until morning," he said, in a speech from the stage which was broadcast on Ukraine's News 24 channel.
"We are talking about human lives and the future of the country which could be drowned in blood. Stop, Viktor Yanukovych, stop."
An aide to Mr Yanukovych was quoted by Ukrainian media earlier as saying the president was "against confrontation and conflict" and did not want to forcibly disperse the camp.
Watching this on TV,and it getting only worse there.
There were protests in my country Bosnia recently, but nothing on this scale of violence. Hope they won't end up in civil war,it would only bring more suffering to them,and even worse situation in world.
Especially becouse of US,EU and Rusia involvment in that part of the world.