Ukrainian protesters have launched a nationwide strike and began to blockade government buildings after violent clashes in which more than 100,000 sought early elections over the authorities’ rejection of a historic EU pact.

Supporters of the ex-Soviet state’s closer alliance with the European Union and disavowal of old master Russia camped out overnight into Monday on Kiev’s iconic Independence Square.

The group huddled in 20 huge military tents to keep warm against the winter chill while musicians performed on a makeshift stage and opposition MPs paid periodic visits to boost morale.

More than 5000 of them then moved toward the government and presidential administration buildings in a bid to force authorities to immediately step down.

Streams of cars honked their horns in support while church bells rang out across the heart of the ancient capital.

The energetic crowd had first defied a ban on protests on Sunday by driving lines of helmeted police off the expansive square in scenes reminiscent of the 2004 pro-democracy Orange Revolution.

Some of the more militant in the group also steered a yellow bulldozer within striking distance of barricades protecting the nearby office of President Viktor Yanukovych.

Security forces outside the president’s seat of power fired dozens of stun grenades and smoke bombs at masked demonstrators who were pelting police with stones and Molotov cocktails.

Kiev police spokeswoman Olga Bilyk said that around 100 officers were wounded in Sunday’s clashes. A mayor’s office official said nearly 50 demonstrators had also been treated by doctors for various injuries.

The economically struggling nation of 46 million was thrown into its deepest crisis in nearly a decade when Yanukovych snubbed EU leaders at a summit on Friday and refused a deal that would have paved Ukraine’s way to eventual membership in the 28-nation bloc.

EU leaders primarily blame the decision on the stinging economic punishments Russia had threatened should Ukraine take the fateful step toward the West.

Yet the move now threatens to backfire on Yanukovych as his political foes try to build momentum amid existing discontent with state corruption and disappearing jobs.

About 50,000 protesters also rallied on Sunday in the Ukrainian-speaking western city of Lviv.

Another 250 EU supporters ignored a court ban in Yanukovych’s native region of Donetsk.

What the opposition describes as the largest Kiev demonstration since the 2004 uprising also saw a few dozen members of the nationalist Svoboda party take control of an empty Kiev city hall building.

“A revolution is starting in Ukraine,” Svoboda party chief Oleh Tyahnybok declared. “We are launching a national strike.”