KIEV (AFP) - Ukrainian riot police approached Kiev's main opposition camp on Tuesday after at least seven people were killed in the bloodiest day of clashes in nearly three months of protests.

Opposition leader Vitali Klitschko called on women and children to quit the encampment on Kiev's iconic Independence Square over fears of a possible police assault. But some 25,000 protestors remained on the square after the expiry of a 6:00pm ultimatum from security forces demanding calm be restored.

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

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.

Ukraine President Viktor 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.

Security forces issued an ultimatum, warning that they would take "grave actions" to restore calm if unrest persisted but no major assault had begun by the time the evening deadline expired. Kiev also shut down the whole of its vast subway network for the first time in the three month-long crisis.

Riot police broke through outlying barricades around the square and massed near the main encampment after they succeeded in forcing protesters back with the help of heavy reinforcements.

Tuesday saw the first violent clashes since mid-January in the Ukrainian capital, which has been wracked by anti-government demonstrations since 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 on 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.

Demonstrators were calling on the Rada Parliament - where Mr Yanukovych's party has a majority - to vote on returning the country to its 2004 constitution, under which key powers would shift from the president to Parliament.

"People were tired of waiting for the constitution to be changed - they needed action," said demonstrator Volodymir, from Kiev, refusing to give his second name.

Another protestor Anatoliy, also from Kiev, said that the latest Tuesday's protests could outstrip January's brutal clashes when several protestors were killed.

"I think the actions will be on a bigger scale than they were on Grushevsky street," where January's fatal clashes happened, he said.

"We need to surround the Parliament until there is a complete change of government."

The political crisis in Ukraine has snowballed into a titanic tug of war for the country's future between Russia and the West.

The US called on Mr Yanukovych to stop the bloodshed and said it was "appalled" by the violence searing Kiev.

"We are appalled by the violence taking place in downtown Kiev. We urge President Yanukovych to immediately de-escalate the situation and end the confrontation at Maidan," said Laura Lucas Magnuson, a spokeswoman for the National Security Council.

The EU's foreign policy chief, Catherine Ashton, said that she was "deeply worried" by the escalation of violence in Kiev and the reported death and casualty toll.

"I urge the leadership of Ukraine to address the root causes of the crisis," Ms Ashton said in a statement.

But the Russian foreign ministry lashed out at Western countries, accusing them of turning a blind eye to the more radical elements inside the protest movement.

"What is happening is a direct consequence of the policy of connivance among those Western politicians and European agencies that have been shutting their eyes to the aggressive actions of Ukraine's radical forces since the beginning of the crisis," the Russian foreign ministry said in a statement.

Russia on Monday said it would release a US$2 billion tranche of a larger US$15 billion bailout that it had essentially frozen since protests escalated last month.

Opposition leaders have called on the EU to slap sanctions on Mr Yanukovych and his backers in a bid to ratchet up pressure on the beleaguered leader.

The latest violence came as Ukraine seemed to be inching towards resolving the turmoil. The authorities on Monday had granted an amnesty to arrested protestors after the opposition vacated Kiev's city hall.