At least 21 civilians were killed in fresh fighting in Kiev on Thursday, shattering an overnight truce declared by Ukrainian President Viktor Yanukovich, and a presidential statement said dozens of police were also dead or wounded.
Activists hurling petrol bombs and paving stones drove riot police off a corner of the central Independence Square, known as the Maidan, and appeared to capture several uniformed officers. Police responded with stun grenades.
The clashes erupted shortly before three visiting European foreign ministers were due to meet the Russian-backed Yanukovich to push for a compromise with his pro-European opponents. The meeting was delayed for security reasons but began an hour late.
A Reuters photographer counted 21 bodies in civilian clothes in three places on the square, a few hundred meters (yards) from the presidency. That raised the death toll since Tuesday to at least 43, by far the bloodiest hours of Ukraine's 22-year post-Soviet history.
A statement from Yanukovich's office said: "They (the protesters) went on to the offensive. They are working in organized groups. They are using firearms, including sniper rifles. They are shooting to kill.
"The number of dead and injured among police officers is dozens," the statement on the presidential website said.
Shortly after 9 a.m. (0700 GMT), the protesters advanced to a line closer to Yanukovich's office and parliament. Television showed activists in combat fatigues leading several captured, uniformed policemen across the square.
Both sides have accused the other of using live ammunition.
The foreign ministers of Germany, France and Poland were expected to present Yanukovich with a mixture of sanctions and enticements to make a deal with his opponents that could end the bloodshed.
"Black smoke, denotations and gunfire around presidential palace ... Officials panicky," tweeted Polish minister Radoslaw Sikorski to explain the delay in the meeting.
Pro-EU activists have been keeping vigil in the square since the president turned his back on a trade pact with the bloc in November and accepted financial aid from Moscow.
Russia, which has been holding back a new loan installment until it sees stability in Kiev, has condemned EU and U.S. support of the opposition demands that Yanukovich, elected in 2010, should share power and hold new elections.
In an apparent criticism of Yanukovich's handling of the crisis, Russian Prime Minister Dmitry Medvedev said on Thursday that Moscow could only cooperate fully with Ukraine when its leadership was in "good shape", Interfax reported.
The crisis in the sprawling country of 46 million with an ailing economy and endemic corruption has mounted since Yanukovich, under pressure from the Kremlin, took a $15-billion Russian bailout instead of a wide-ranging deal with the EU.
The United States stepped up pressure on Wednesday by imposing travel bans on 20 senior Ukrainian officials, and European Union foreign ministers are due to meet in Brussels later on Thursday to consider similar measures.
A statement on Yanukovich's website announced an accord late on Wednesday with opposition leaders for "the start to negotiations with the aim of ending bloodshed, and stabilizing the situation in the state in the interest of social peace".
Responding cautiously, U.S. President Barack Obama deemed the truce a "welcome step forward", but said the White House would continue to monitor the situation closely to "ensure that actions mirror words".
"Our approach in the United States is not to see these as some Cold War chessboard in which we're in competition with Russia," Obama said after a North American summit in Mexico.
"Our goal is to make sure that the people of Ukraine are able to make decisions for themselves for the future," he said.
At the Winter Olympics in Sochi, being hosted by Russia, some members of Ukraine's team have decided to leave, the International Olympic Committee said on Thursday.
Protesters were in a truculent mood despite the overnight lull and columns of men, bearing clubs and chanting patriotic songs headed to Independence Square at 8:30 a.m. (0630 GMT).
"What truce? There is no truce! It is simply war ahead of us! They are provoking us. They throw grenades at us. Burn our homes. We have been here for three months and during that time nothing burned," said 23-year-old Petro Maksimchuk.
"These are not people. They are killers. Sanctions will not help. They all should be sent into isolation in Siberia."
Serhiy, a 55-year-old from the western city of Lviv who declined to give his surname, added: "It is bad that Ukraine is already broken into two parts. In the west the police and army are with us but in the east, they are against us.
"It is the 'Yanukovichers' who are dividing us."
In Lviv, a bastion of Ukrainian nationalism since Soviet times, the regional assembly declared autonomy from Yanukovich and his administration, which many west Ukrainians see as much closer to Moscow and to Ukraine's Russian-speaking east.
The health ministry revised upwards the death toll in Tuesday's violence to 28 as of 6 a.m. on Thursday - three hours before the latest violence and the 15 dead counted by Reuters.
Yanukovich, who replaced the head of the armed forces, had denounced the bloodshed in central Kiev as an attempted coup. His security service said launched a nationwide "anti-terrorist operation" after arms and ammunition dumps were looted.
Reflecting Western outrage at the crackdown, EU ambassadors discussed a series of possible steps including asset freezes and travel bans, even though diplomats doubt the are effective.
Jumping out ahead of its EU allies, Washington imposed U.S. visa bans on 20 government officials it considered "responsible for ordering human rights abuses related to political oppression", a State Department official said.
"These individuals represent the full chain of command we consider responsible for ordering the security forces to move against" the protesters, the official said.
EU officials said Yanukovich himself would be excluded from such measures in order to keep channels of dialogue open.
As well as asset freezes and visa bans, ministers will discuss measures to stop riot gear and other equipment being exported to Ukraine and could consider arms restrictions.
Diplomats said the threat of sanctions could also target assets held in the West by Ukrainian business oligarchs who have either backed Yanukovich or are sitting on the fence.
The United States, going head to head with Russia in a dispute reminiscent of the Cold War, urged Yanukovich to pull back riot police, call a truce and talk to the opposition.
Obama warned the Ukrainian armed forces that the crackdown could damage "our defense relationship". But Washington appears to have little direct leverage in Kiev.
Russian President Vladimir Putin, who has met Yanukovich six times since the crisis began, has kept quiet on the flare-up. But Foreign Minister Sergei Lavrov blamed the West for encouraging opposition radicals "to act outside of the law."
Moscow said on Monday it would resume stalled aid to Kiev, pledging $2 billion just hours before the crackdown began. The money has not yet arrived, and a Ukrainian government source said it had been delayed until Friday "for technical reasons."
Putin's spokesman said on Thursday that Moscow was waiting for the situation to normalize before paying up.
Ukraine's hryvnia currency, flirting with its lowest levels since the global financial crisis five years ago, weakened again on Thursday.
Possibly due to the risk of sanctions, three of Ukraine's richest entrepreneurs have stepped up pressure on Yanukovich to hold back from using force.
"There are no circumstances which justify the use of force toward the peaceful population," steel and coal magnate Rinat Akhmetov said in a statement late on Tuesday.
Akhmetov, who partly bankrolled Yanukovich's election campaign in 2010 and whose wealth is put by Forbes at more than $15 billion, said: "People's deaths and injuries on the side of protesters and the security forces in street battles are an unacceptable price for political mistakes."
Viktor Pinchuk, another steel billionaire known in the West for his philanthropic activity, said: "A peaceful solution must be found. It is imperative to refrain from the use of force and find a compromise."
Dmytro Firtash, a gas and chemicals magnate who is part owner of a popular TV channel, said in a statement: We, through our joint actions, must end the bloodshed. We are against radical actions by whomever it might be."