Ukrainian protesters will go "on the attack" if President Viktor Yanukovych does not swiftly offer concessions, opposition leader Vitali Klitschko has pledged.

After fierce clashes on Wednesday, Klitschko told tens of thousands of protesters crammed into Kiev's Independence Square that Yanukovych could resolve the situation without bloodshed by calling early elections.

"If Yanukovych does not make concessions, then tomorrow [Thursday] we will go on the attack," Klitschko said.

His comments came as at least three demonstrators were killed - the first fatalities in two months of anti-government protests.

Pitched battles raged in the centre of the Ukrainian capital as protesters hurled stones and Molotov cocktails at police and security forces responded with tear gas, stun grenades and rubber bullets.

Ukraine's prime minister has taken a hard line against demonstrators flouting anti-protest laws, saying "terrorists" had taken to the streets of Kiev and that opposition leaders should be held responsible for the deaths.

"I am officially stating that these are criminals who must answer for their action," Mykola Azarov said.

He said the police did not have live ammunition.

Kiev standoff

The country has been gripped by protests since Yanukovych rejected a deal with the European Union and instead signed a pact with Russia.

For the past two nights, demonstrators and police have been clashing in Kiev's city centre.

Two of the victims on Wednesday died from gunshot wounds. The third died in a fall from the top of a football stadium.

The bodies were found before police moved to tear down protesters' barricades near official buildings in central Kiev and chase demonstrators away.

Helmeted riot police moved in on hundreds of protesters, dismantling barricades, beating many with truncheons and firing shots at some.

The police drove demonstrators down a hill towards the main protest site on Independence Square, where protesters have set up an extensive tent camp and rallied around the clock since November 21.

But the protesters soon returned, building barricades from giant sacks of snow and hurling stones and firebombs at police lines. There was no immediate police move on the main camp.

Controversial new laws essentially ban large protests in Ukraine, a move that has sparked criticism from the UN's High Commissioner for Human Rights.

Amid growing international concern, EU foreign policy chief Catherine Ashton urged "an immediate end" to the escalating violence.

European Commission president Jose Manuel Barroso warned the authorities that the EU executive authority would assess "possible actions" against the Ukrainian authorities.