Pro-European Ukrainian demonstrators took control Sunday of the Kiev mayor's office and set up what they described as the temporary headquarters of the united opposition, a police spokeswoman told AFP.

"Law enforcement officials are now negotiating with those who took control of the mayor's office. They are being told that what they did was illegal and being asked to leave the building," Kiev police spokeswoman Olga Bilyk said by telephone.

Bilyk also told AFP that around 100 Ukrainian police officers were injured in violent  protests that broke out Sunday outside President Viktor Yanukovych's office in central Kiev.

"Around 100 law enforcement officials were injured in the Kiev protests," she said by telephone.

Some 100,000 Ukrainians chanting "Revolution!" swarmed a central Kiev square Sunday in a mass call for early elections meant to punish President Yanukovych for rejecting a historic EU pact.

The crowd, with many waving the gold-and-blue flag of the European Union, took control of Kiev's iconic Independence Square and steered a bulldozer within striking distance of metal police barricades protecting the presidential administration building on a nearby side street.

Footage broadcast by a private Ukraine television station also showed a few dozen people breaking into an empty Kiev city hall building and reading speeches demanding an immediate break in relations with Russia.

"The government and president must resign," world boxing champion turned opposition leader Vitali Klitschko said to loud cheers and cries of "Yes!"

"A revolution is starting in Ukraine," added nationalist opposition leader Oleh Tyagnybok.

"We are now launching a national strike," he said in dramatic scenes aired live on television stations in both Ukraine and Russia.

The ex-Soviet nation of 46 million was thrown into its deepest crisis since the 2004 pro-democracy Orange Revolution when Yanukovych snubbed EU leaders at a Vilnius summit on Friday and opted to keep Ukraine aligned with its former master Russia.

The government's decision, first announced a week before the EU meeting, sparked mass demonstrations that turned violent in the early hours of Saturday when hundreds of rubber baton-wielding police drove about 1,000 protesters from Kiev's focal Independence Square.

A few hundred of them spent the night at the nearby Mikhailovsky Monastery, burning wood in metal barrels to ward off the freezing temperature and receiving food from the monastery's monks.

The protesters vowed to form a "national resistance task force" and called for early elections as well daily rallies aimed at blocking the entrance to the Ukrainian government seat.

Sunday's demonstration was held in defiance of a sudden ban imposed late Saturday by Kiev's main administrative court on all protests on the square and its surrounding streets until January 7.