Dozens of protesters were reported killed by government snipers in the Ukrainian capital Kiev after they had hurled firebombs and advanced on police lines.
At least 70 died and hundreds were wounded according to an unconfirmed report from a protest doctor.
Video footage on Ukrainian television showed shocking scenes today of protesters being cut down by gunfire, lying on the pavement as comrades rushed to their aid. Trying to protect themselves with shields, teams of protesters carried bodies away on sheets of plastic or planks of wood.
One of the wounded, volunteer medic Olesya Zhukovskaya, sent out a brief Twitter message - "I'm dying" - after being shot in the neck. Dr Oleh Musiy, the medical coordinator for the protesters, said she was in serious condition after being operated on.
Protesters were also seen leading policemen with their hands held high around the sprawling protest camp in central Kiev. Ukraine's Interior ministry says 67 police were captured in all - it was not clear how. An opposition MP said they were being held in Kiev's occupied city hall.
President Viktor Yanukovych and the opposition protesters who are demand his resignation are locked in an epic battle over the identity of Ukraine, a nation of 46 million that has divided loyalties between Russia and the West. Parts of the country - mostly in its western cities - are in open revolt against Yanukovych's central government, while many in eastern Ukraine favour strong ties with Russia, their former Soviet ruler.
Protesters are also upset over corruption, the lack of democratic rights and the country's ailing economy, which just barely avoided bankruptcy with a 15 billion dollar (£9bn) loan from Russia.
In Brussels, the European Union decided in an emergency meeting today to impose sanctions against those behind the violence in Ukraine. The US is considering whether to join the EU sanctions. A freeze on assets and travel bans could hurt the oligarchs who back Yanukovych.
At least 101 people have died this week in the clashes in Kiev, a sharp reversal in three months of mostly peaceful protests. Now neither side appears willing to compromise or in control of the streets. The opposition insisting on Yanukovych's resignation and an early election while the embattled president is apparently prepared to fight until the end.
Today was the deadliest day yet at the sprawling protest camp on Kiev's Independence Square, also called the Maidan. Snipers were seen shooting at protesters there - and video footage showed at least one sniper wearing a Ukraine riot police uniform.
Musiy, the protest doctor, said at least 70 protesters were killed today and over 500 were wounded in the clashes - and that the death toll could well rise further.
In addition, three policemen were killed and 28 suffered gunshot wounds, said Interior Ministry spokesman Serhiy Burlakov.
There was no way to immediately verify any of the death tolls. Earlier in the day, an Associated Press reporter saw the bodies of 21 protesters laid out near Kiev's protest camp.
President Barack Obama said the US was outraged by the violence and issued a statement urging Yanukovych to withdraw his forces from central Kiev immediately. He also said Ukraine should respect the right of protest and that protesters must be peaceful.
Although the first weeks of the protests were determinedly peaceful, radical elements have become more influential as impatience with the lack of progress grows. In their battles today, protesters regained some of the territory on the Maidan's fringes that police had seized earlier in the week.
One camp commander, Oleh Mykhnyuk, said protesters threw firebombs at riot police on the square overnight. As the sun rose, police pulled back, the protesters followed them and police then began shooting at them, he said.
The Interior Ministry warned Kiev residents to stay indoors because of the "armed and aggressive mood of the people".
Yanukovych claimed today that police were not armed and "all measures to stop bloodshed and confrontation are being taken." But the Interior Ministry later contradicted that, saying law enforcers were armed as part of an "anti-terrorist" operation.
Some signs emerged that Yanukovych is losing loyalists. The chief of Kiev's city administration, Volodymyr Makeyenko, announced he was leaving Yanukovych's Party of Regions.
"We must be guided only by the interests of the people, this is our only chance to save people's lives," he said, adding he would continue to fulfill his duties as long as he had the people's trust.
Another influential member of the ruling party, Serhiy Tyhipko, said both Yanukovych and opposition leaders had "completely lost control of the situation".
"Their inaction is leading to the strengthening of opposition and human victims," the Interfax news agency reported.
The parliament building was evacuated because of fears that protesters would storm it, as were the government office in Kiev and the Foreign Ministry buildings. But parliament convened in the afternoon, with some pro-government MPs heeding the opposition's call to work out a solution to the crisis.
As the violence exploded today and heavy smoke from burning barricades at the encampment belched into the sky, the foreign ministers of three EU countries - France, Germany and Poland - met with Yanukovych for five hours after speaking with the opposition leaders. The EU ministers then returned to speak again with opposition leaders.
Before the clashes today, the Ukrainian Health Ministry said 28 people have died and 287 were taken to hospital this week. Protesters who have set up a medical facility in a city cathedral so that wounded colleagues would not be snatched away by police say the number of injured are significantly higher - possibly double or triple that.
The Caritas Ukraine aid group praised the protest medics but said many of the wounded will need long-term care, including prosthetics.
The clashes this week have been the most deadly since protests kicked off in November after Yanukovych shelved an association agreement with the European Union in favour of closer ties with Russia. Russia then announced a 15 billion dollar (£9bn) bailout for Ukraine.
In Moscow, the Kremlin said President Vladimir Putin was sending former ombudsman Vladimir Lukin to Ukraine to act as a mediator.
Prime Minister Dmitry Medvedev said Russia will "try to do our best" to fulfill its financial obligations to Ukraine, but indicated Moscow would hold back on further bailout installments until the crisis is resolved.
"We need partners that are in good shape and a Ukrainian government that is legitimate and effective," he said.
At the Winter Olympics in Sochi, Ukrainian alpine skier Bogdana Matsotska, 24, said she will not take part in tomorrow's women's slalom due to the developments in Kiev.
"As a protest against lawless actions made toward protesters, the lack of responsibility from the side of the president and his lackey government, we refuse further performance at the 2014 Sochi Olympic Games," her father and coach, Oleg Matsotskyy, wrote in a Facebook post.