At least 70 people have reportedly been killed after police fired live rounds at anti-government protesters near Independence Square in Ukraine's capital Kiev.
The claim was made by doctors working with the activists, and cited by news agencies. Hundreds more demonstrators have been injured.
Meanwhile, protesters have taken 67 police officers hostage, according to the interior ministry.
Protesters have reclaimed and expanded territory in the center of Kiev that they had lost two days earlier when police launched an unsuccessful assault on Independence Square.
Sky News's David Bowden said protesters pushed their way up a street towards the government district after the truce between both sides crumbled, and police "scurried away" to the top of the street in panic, before launching a counter attack.
He said: "Police are hitting back and are shooting – probably not at random – but they are shooting with live rounds at the protesters. The police seem to have been caught off guard and they're reacting very aggressively and basically just shooting people.”
"There was, for a brief point, a sniper with a sniper rifle on a tripod and a spotter picking off protesters for a while."
Bowden observed activists building a barrier about 12-15 ft high and "moving piles of tires to create a big smokescreen because they fear the police will try and pick them off one by one with snipers on rooftops further up the road".
The fresh violence comes after President Viktor Yanukovych announced a truce with the opposition, following violent clashes that killed at least 26 people on Wednesday.
A growing number of once loyal members of the ruling Party of Regions, including the mayor of Kiev, announced they were quitting the party to protest the bloodshed.
Some Ukrainian athletes have left the Sochi Winter Olympics, an IOC spokesman said.
EU foreign ministers are now meeting to discuss the violence.
Russia has warned Ukraine's president not to let opponents walk over him "like a doormat", and has said that threats of sanctions from the West are "inappropriate.”
British Foreign Secretary William Hague called the Ukraine government's actions "utterly indefensible.”