"The National Security Service and Ukrainian Anti-Terrorist Centre have taken the decision to conduct an anti-terrorist operation across the territory of Ukraine," security agency head Oleksandr Yakimenko said in a statement.

Embattled President Viktor Yanukovych was informed of the operation that will see security agencies work with the ministry of defence to strengthen state borders and maintain public order, the statement said.

"Radical and extremist groups are posing a real threat to the lives of millions of Ukrainians with their actions," Yakimenko said.

The statement said that over the past few days some 1,500 firearms and 100,000 bullets had ended up in the hands of "criminals".

The statement said that Ukraine was faced with "concrete acts of terrorism".

"Today we are witnessing the use of deliberate and targeted force through arson, murder, hostage-taking and intimidation of the population for criminal ends," it said.

Protesters remained locked in a tense standoff on Wednesday after riot police halted an assault against Kiev's main protest camp on the iconic Independence Square following the bloodiest day of violence in almost three months of demonstrations.

Opposition leaders refused to back down and repeated demands for Yanukovych to leave power.

"The government of Yanukovych has to resign. Only Yanukovych is responsible for murder and terror against peaceful civilians," opposition leader and former heavyweight boxer Vitali Klitschko said.

"Blood is spilled on the streets of Kiev. The main reason for this is actions of the authorities," Klitschko said.


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