Ukraine pro

After the violent scenes of a police crackdown in the early hours of Saturday, demonstrators returned with a vengeance today, angry at their president's refusal to sign an agreement with the European Union.

Thousands broke away from the main rally in a bid to storm the nearby presidential building, armed with a heavy-duty front loader truck, but were driven back by riot police using tear gas and stun grenades.

In another part of the city centre, a group of protesters broke through windows and locked doors to storm the headquarters of the city council. Once inside, the hung a Ukrainian flag in the window and chanted 'Kiev is ours'.

The world heavyweight boxing champion Vitaly Klitschko, who leads the opposition Udar party, called for calm.

He denounced the attempt to storm the president's office, describing it as an effort to make the government declare a state of emergency, and instead called for an ongoing peaceful protest in the square.

With such numbers attending, officers had no choice but to allow the central rally to proceed in peace, though there were clashes with police lines on the fringes. Prominent opposition leaders called for calm and distanced themselves from the more violent elements of the demonstration.

The Interior Ministry said the confrontation was initiated by about 200 people wearing masks, who commandeered a front loader to try to break through police lines and also used gas against police.

Speaking at the demonstration from the roof of a bus, the opposition leaders demanded that President Viktor Yanukovych and his government resign. They also called for a nationwide strike and for tents to be set up to allow demonstrators to remain on the square around the clock.

'Our plan is clear: It's not a demonstration, it's not a reaction. It's a revolution,' Yuriy Lutsenko said.

Chants of 'revolution' resounded across a sea of yellow and blue Ukrainian and EU flags on the square, where the government had prohibited rallies starting Sunday.

The crowd was by far the largest since the protests began more than a week ago. Many of the demonstrators had traveled to Kiev from western Ukraine, where pro-EU sentiment is particularly strong.

'We are furious,' 62-year-old retired businessman Mykola Sapronov said. 'The leaders must resign. We want Europe and freedom.'

Protests have been held daily in Kiev since Yanukovych backed away from an agreement that would have established free trade and deepened political cooperation between Ukraine and the EU. He justified the decision by saying that Ukraine couldn't afford to break trade ties with Russia.

The EU agreement was to have been signed Friday and since then the protests have gained strength.

Sunday's demonstration also was energized by anger over the violent dispersal of several hundred protesters at Independence Square early Saturday. Some of the protesters were left bleeding from their heads after riot police beat them with truncheons.

'They want to take our freedom away from us,' said Nina Moskalik, 25. 'They beat people, they spill blood. This is why we have to come out.'

Yanukovych late Saturday condemned the use of force and promised to punish those responsible.

As the vast column of demonstrators approached Independence Square on Sunday and swept away metal barriers from around a large Christmas tree set up in the center, all police left the square. About a dozen demonstrators then climbed the tree to hang EU and Ukrainian flags from its branches.

Several hundred demonstrators never made it to the square. Along the way they burst into the Kiev city administration building and occupied it, in defiance of police, who tried unsuccessfully to drive them away by using tear gas.

The EU agreement had been eagerly anticipated by Ukrainians who want their country of 45 million people to break out of Moscow's orbit. Opinion surveys in recent months showed about 45 percent of Ukrainians supporting closer integration with the EU and a third or less favoring closer ties with Russia.

Moscow tried to block the deal with the EU by banning some Ukrainian imports and threatening more trade sanctions. A 2009 dispute between Kiev and Moscow on gas prices resulted in a three-week cutoff of gas to Ukraine.

Yanukovych was traveling to China for a state visit this week. Afterward, the president planned to visit Russia and reach agreement on normalizing trade relations, Prime Minister Mykola Azarov said Sunday.

Sunday's demonstration was the largest since the mass protests of the 2004 Orange Revolution, when tens of thousands came to the square nightly for weeks and set up a vast tent camp on the main street leading to the square.

Those protests forced the annulment of a fraud-tainted presidential election in which Yanukovych was shown with the most votes. A rerun of the election was ordered, and Yanukovych lost to Western-leaning reformist Viktor Yushchenko.

Yanukovych was elected president five years later, narrowly defeating then-Prime Minister Yulia Tymoshenko, the leading figure of the Orange Revolution.

Tymoshenko was sentenced to seven years imprisonment in 2011 for abuse of office, a case that the West has widely criticized as political revenge. The EU had set Tymoshenko's release, or at least her freedom to go to Germany for treatment of a severe back problem, as a key criterion for signing the association pact with Ukraine.

The prospect of freeing his archenemy was deeply unattractive to Yanukovych, who comes up for re-election in early 2015.

Additional reporting by AP