Ukrainian riot police continued a massive assault on anti-government protesters in central Kiev early Wednesday, after the bloodiest outburst of violence in nearly three months of demonstrations.

The police began to surround the Independence Square, the main site where protesters had camped, at around 8 p.m. local time (1800 GMT) Tuesday, dismantling barricades and dispersing demonstrators with stun grenades and water cannon.

At least 14 protesters and seven policemen were reportedly killed during violence that erupted Tuesday and continued into the early hours of Wednesday.

The clashes also left hundreds of people from both sides of the conflict injured, with dozens of them in a serious condition, said police and opposition representatives.

On Tuesday morning, anti-government protesters attacked police with Molotov cocktails outside the parliament building. Two vehicles blocking the way to the parliament were torched by the demonstrators.

The activists also set on fire the headquarters of the ruling Party of Regions in downtown Kiev. The police responded with stun grenades and tear gas to push the crowd back.

Tuesday's unrest marked the latest wave of clashes between demonstrators and riot police in the political standoff, which began last November when the Ukrainian government's decision to backtrack on the country's European integration angered many Ukrainians.

The demonstrations turned violent on Jan. 19, when radical activists attacked riot police with fireworks and petrol bombs, causing casualties.

In New York, UN spokesperson Martin Nesirky said at a daily news briefing that the chief of the world body expressed grave concern about renewed violence and fatalities in Kiev.

UN Secretary-General Ban Ki-moon closely followed the developments in Ukraine and "reiterates his appeal to all concerned to act with restraint in order to avoid any further violence," Nesirky said.

US Vice President Joe Biden also called on Ukrainian President Viktor Yanukovych to pull back government forces and exercise "maximum" restraint in the current crisis.

Biden called Yanukovych on Tuesday to express "grave" concern regarding the crisis on the streets of Kiev, said a statement issued by the White House, adding that the vice president underscored the urgency of immediate dialogue with opposition leaders.

In Paris, French Foreign Minister Laurent Fabius condemned the fresh wave of violence in Ukraine, urging all parties to exercise restraint and resume dialogue.

"I condemn the resumption of violence in Kiev and the indiscriminate use of force which resulted in several deaths. I call on all parties to exercise the utmost restraint and to immediately resume dialogue," Fabius said in a statement.

"It (the dialogue) is the only way to reach a political solution that can meet the aspirations of the Ukrainian people," he said.

Meanwhile, Ukrainian opposition leader Vitali Klitschko said Yanukovych has told opposition leaders that he will not call off the police assault on the opposition's main protest camp on the Independence Square.

"Yanukovych said that there is only one option...to clear Maidan and that everyone has to go home," Klitschko told independent Hromadske TV after meeting the president.

Responding to concerns and criticisms, Ukraine's Acting Foreign Minister Leonid Kozhara asked the international community to make an objective evaluation of the recent violence.

"We call on foreign governments and international organizations to make a fair and impartial assessment of the situation in Ukraine," Kozhara was quoted by the Foreign Ministry's press office as saying.

He voiced his hope that the international community would "strongly condemn" the illegal actions of radical activists, who initiated a new wave of "violence and lawlessness" in Ukraine.