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 13 people were killed, including six policemen, in the worst violence seen in weeks. Two key protest leaders have gone to see President Viktor Yanukovych, who has so far refused calls to resign. Vitali Klitschko, the pro-EU leader of the Udar (Punch) movement, and Fatherland leader Arseniy Yatsenyuk, arrived at Mr Yanukovych’s residence from the Maidan late on Tuesday, but it was not clear whether they had yet seen him.
Both leaders have so far refused to end the protests.
[color=rgb(80,80,80)]‘Island of freedom’[/color]
Security forces had given protesters a deadline of 18:00 local time (16:00 GMT) to end the unrest and had surrounded Independence Square, 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.
Then shortly before 18:00 GMT, police announced over loudspeakers that they were about to begin “an anti-terror operation”.
Protesters have insisted they will not leave the Maidan
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 block off police. Many tents have been burned but it was unclear whether there had been casualties.
In speeches from the main stage, protest leaders urged people to stay.
“We will not go anywhere from here,” said Mr Klitschko. “This is an island of freedom and we will defend it.”
Mr Yatsenyuk appealed to President Yanukovych to “stop the bloodshed and call a truce”.
“We are talking about human lives and the future of the country which could be drowned in blood. Stop, Viktor Yanukovych, stop,” he said, in comments broadcast live on Ukraine’s News 24 channel.
[color=rgb(51,51,51)]Former deputy prime minister and activist Oleg Rybachuk: “The people are surrounded”[/color]
Hours later, the police action appeared to have been scaled down, and large numbers of people were still in the square.
Meanwhile, there are reports of unrest breaking out elsewhere in Ukraine, including the western cities of Lviv and Ivano-Frankivsk.
The BBC’s David Stern in Kiev says this is a key moment for the country and that many people are scared of further escalation. Although this does not necessarily mean a civil war – as some have previously suggested – Ukraine remains dangerously divided, our correspondent says.
Ukraine’s unrest began in November, when President Yanukovych rejected a trade deal with the EU in favour of closer ties with Russia.
Pro-EU protesters demanded his resignation and snap elections.
After weeks of unrest, the mood had calmed in recent days, but people remained on the streets,
Earlier on Tuesday, police blocked protesters from marching on parliament, where MPs had been due to debate proposed changes to the constitution which would have reduced the powers of the president.
The debate did not take place. Mr Yatsenyuk said President Yanukovych was blocking the reforms and that his allies “show no desire whatsoever to end the political crisis”.
But MPs who support the president said the proposals had not been thoroughly discussed, and that more time was needed.
Some protesters outside parliament ripped up cobblestones to throw at police. Police fired stun and smoke grenades, and rubber bullets. Correspondents say it was unclear what sparked the latest violence, with each side blaming the other.
Protesters also attacked the headquarters of President Yanukovych’s Party of the Regions, temporarily smashing their way in and setting it on fire before being forced out by police.
Police said late on Tuesday that at least 13 people were had been killed, including six police officers.
The White House said it was “appalled” by the violence, saying “force will not resolve the crisis”.
Spokesman Jay Carney urged President Yanukovych to “restart a dialogue with opposition leaders today”.