KIEV: Ukrainian riot police stormed the main opposition camp in Kiev Tuesday after clashes left at least seven dead in the bloodiest day in three months of protests, triggering international alarm.

Opposition leader Vitali Klitschko called on women and children to quit the encampment on Kiev's iconic Independence Square as riot police began their assault.

But some 25,000 protestors remained on the square after the expiry of a 6:00pm (1600 GMT) ultimatum from security forces demanding calm be restored.

Police said five civilians and two policemen had died during a day of clashes that turned parts of central Kiev into a war zone.

Protesters seized back control of Kiev's city hall, with around 30 activists setting up a first aid point inside the building, which the opposition had left on Sunday as part of an amnesty deal with the authorities, according to AFP reporters on the scene.

The European Union, UN, US and Nato all voiced concern over the clashes, with EU foreign policy chief Catherine Ashton saying she was “deeply worried about the grave new escalation”.

UN chief Ban Ki-moon called for restraint and dialogue, while Washington said it was “appalled” by the violence and said Ukraine's President Viktor Yanukovych must “de-escalate the situation”.

Nato head Anders Fogh Rasmussen said he was “seriously concerned” and urged “all parties to refrain from violence and to urgently resume dialogue, including through the parliamentary process”.

Smoke bombs and stun grenades

Tuesday saw the first violent clashes since mid-January in the Ukrainian capital, which has been wracked by anti-government demonstrations since President Viktor Yanukovych in November rejected an EU pact in favour of closer ties with historical master Russia.

Protesters briefly seized Yanokuvych's party headquarters after several hundred attacked it with Molotov cocktails and smashed their way inside but later withdrew as smoke continued to billow from part of the building, an AFP reporter at the scene said.

Fighting flared Monday morning after some 20,000 mainly peaceful protesters marched from their sprawling tent encampment towards parliament to demand legislators strip the president of a raft of powers.

Police fired rubber bullets and hurled smoke bombs and stun grenades at protesters who threw paving stones and set two trucks on fire while trying to break through to the heavily-fortified parliament.

German Foreign Minister Frank-Walter Steinmeier warned that the clashes could lead to EU sanctions against individuals responsible for the violence.

“Whoever is responsible for the decisions which has led to the bloodshed in Kiev and other parts of Ukraine should expect Europe to reconsider its position on imposing sanctions on individuals,” Steinmeier said in a statement.

The EU's enlargement commissioner Stefan Fuele, who has been heavily involved in trying to defuse the Ukraine crisis, said the country's acting premier Sergei Arbuzov had told him the government would do all it could to keep the weapons on the street “silent”.

“I was just on the phone with ... Arbuzov telling him that seeing in the streets Berkut, the special police, with Kalashnikov is a source of great concern,” Fuele told journalists in Brussels.

“He assured me that he and the government would do what they can to make sure that those weapons stay silent. For the sake of Ukraine and its future I will pray that he is right.”

Medics working at field hospitals run by the opposition earlier said that three protesters died of gunshot wounds and that around 150 others were injured, of which some 30 were in a serious condition.

Yanukovych's ruling Regions Party said that an employee at its headquarters was also found dead after protesters briefly seized the building.

One policeman died while being taken to hospital after being shot in the neck, the interior ministry said. Kiev police later said that a second policeman had died from gunshot wounds. Some 47 other policemen were wounded, authorities said.

'Civil war'

A top Russian lawmaker said Ukraine was on the brink of a civil war that has been inflamed by the West.

Alexei Pushkov, the head of the lower house's foreign affairs committee, described the violence as an attempt to “seize power through chaos and lawlessness” in comments to the Interfax news agency.

“I consider that a significant amount of responsibility for this falls on the West and Western politicians, who are constantly putting pressure on the Ukrainian authorities,” Pushkov said.

Ukraine's prosecutor-general warned he would seek the “harshest punishments” for those behind the outburst of violence.

“The prosecutor-general will demand the harshest punishments for those who provoked people to carry out today's actions, those who organised it and those who directed it,” prosecutor-general Viktor Pshonka said in a statement.