Three hours of fierce fighting in Independence Square, which was recaptured by anti-government protesters, left the bodies of more than 20 civilians strewn on the ground, a few hundred yards from where the president met the EU delegation.
Riot police were seen shooting from a rooftop at demonstrators and Kiev's health department said 67 people had been killed since Tuesday, which meant at least 39 died in yesterday's clashes. It was by far the worst violence since Ukraine emerged from the crumbling Soviet Union 22 years ago.
The foreign ministers of Germany, France and Poland met with Mr Yanukovich for four hours and extended their stay to put a roadmap for a political solution to opposition leaders.
Meanwhile their EU colleagues agreed at an emergency meeting in Brussels to move ahead with sanctions which will include visa bans and asset freezes on those deemed responsible for the violence.
In a sign of dwindling support for Mr Yanukovich, his hand-picked head of Kiev's city administration quit the ruling party in protest at bloodshed in the streets.
But core loyalists were still talking tough. Interior Minister Vitaly Zakharchenko, wearing camouflage, said police had been issued with combat weapons and would use them "in accordance with the law" to defend themselves - or to free 67 of their colleagues his ministry said were being held captive.
Russia hit back at European and US criticism, calling the threat of sanctions "blackmail" which would only make matters worse.
President Vladimir Putin dispatched an envoy to Kiev to join the mediation effort with the opposition at Mr Yanukovich's request.
German Chancellor Angela Merkel called Mr Yanukovich to urge him to accept the offer of EU mediation in the crisis.
The bloodshed yesterday morning, in which both sides used firearms, traumatised many Ukrainians, whose 2004-05 Orange Revolution for democracy passed off largely peacefully.
It heightened concern voiced by Polish Prime Minister Donald Tusk that Ukraine could descend into civil war or split between the pro-European west and Russian-speaking east.
Riot policemen were filmed firing bursts from automatic rifles as they covered retreating colleagues. An opposition militant in a helmet was filmed firing from behind a tree.
Other protesters used police riot shields for cover, while some fell wounded as the protest camp became a killing zone. A presidential statement said dozens of police were wounded or killed during the opposition offensive, hours after Mr Yanukovich and opposition leaders had agreed on a truce.
The interior ministry advised citizens to avoid central Kiev because of the danger from "armed and aggressive individuals". Schools, restaurants and many shops in the normally bustling city of three million were closed, the metro was shut down and bank machines were running out of cash.
A statement from Mr Yanukovich's office said organised gangs of protesters were using firearms, including sniper rifles.
Opposition leader Vitaly Klitschko urged lawmakers to convene in parliament and demanded Mr Yanukovich call an immediate presidential election.
The former boxing world champion said: "The authorities are resorting to bloody provocations in full view of the world."
The crisis in the sprawling country of 46 million with an ailing economy and endemic corruption has mounted since Mr Yanukovich accepted a £12 billion Russian bailout instead of signing a trade deal with the EU. Russia has held back a new loan instalment until it sees stability in Kiev and has condemned EU and US support of the opposition demands that Mr Yanukovich, elected in a broadly fair vote in 2010, should share power and hold new elections.
Some members of Ukraine's team decided to leave Russia's Winter Olympics in Sochi because of the violence back home.
In Lviv, a bastion of Ukrainian nationalism since Soviet times, the regional assembly declared autonomy from Mr Yanukovich and his administration, which many west Ukrainians see as much closer to Moscow and to Ukraine's Russian-speaking east.
Diplomats said the threat of sanctions could also target assets held in the West by Ukrainian business oligarchs who have either backed Mr Yanukovich or are sitting on the fence.