KIEV, Ukraine (CNN) -- A short-lived truce crumbled Thursday as gunfire erupted at Independence Square, the center of anti-government protests and an increasingly violent crisis that threatens catastrophe for this divided nation.
The head of the protesters' medical service said 100 people died and 500 were injured in clashes on Thursday, just hours after the government declared a truce in fighting that had shocked world leaders with the deaths of 28 people two days before.
The Ukrainian government has not released its own casualty figures, but Interior Minister Vitali Zakharchenko said 25 police officers had been wounded and an unspecified number of them had died. Some others had been taken hostage, he said.
If Thursday's death toll is confirmed, it would make it the deadliest day in Ukraine since protests began.
The violence inflames a crisis that started in November, when President Viktor Yanukovych reversed a decision to sign a trade deal with the European Union and instead turned toward Russia. Ukraine has long been divided between historic loyalties to Europe and its eastern neighbor.
It's unclear what prompted Thursday's clashes. CNN crews at the scene reported that as security forces were moving away from the area, a group of protesters pursued them throwing rocks and Molotov cocktails.
Zakharchenko said the violence had been "provoked exclusively by the opposition leaders," echoing an earlier statement from Yanukovych's office accusing protesters of breaking the truce.
"The opposition used the negotiation period to buy time, to mobilize and get weapons to protesters," the statement from the President's office said.
However, a doctor volunteering to treat protesters, Olga Bogomolets, accused government forces of shooting to kill, saying she had treated 13 people she believed had been targeted by "professional snipers."
"They were shot directly to their hearts, their brain and to their neck," she said. "They didn't give any chance to doctors, for us, to save lives."
CNN could not independently confirm Bogomolets' claim of sniper fire.
At a hotel converted into a triage center, bodies covered in bloodied sheets lay on the floor. Orthodox priests prayed over them.
Diplomatic efforts under way
Beyond Kiev's chaotic center, European Union officials were scheduled to meet Thursday in an urgent session to discuss possible sanctions against the Ukrainian government, which could include freezing assets and restricting the visas of officials deemed responsible for violence in that country, French Foreign Minister Laurent Fabius said on Twitter.
He was in Ukraine's capital Thursday, along with foreign ministers from Germany and Poland, meeting with various opposition leaders and Yanukovych ahead of the emergency session in Brussels.
German Chancellor Angela Merkel urged the Ukrainian President to accept European help in talks with the anti-government opposition, Merkel's office said Thursday.
Russia's foreign ministry appeared to criticize Western diplomatic efforts, according to a report by Russian state news agency RIA Novosti.
"The ongoing attempts to obtrusively intervene from outside, threat with sanctions or trying to influence the situation in any other ways are inappropriate and can't lead to anything good but can only aggravate the confrontation," the report quoted spokesman Aleksandr Lukashevich.
As the protests continued, anger over the rejected trade deal with the EU morphed into resentment of Yanukovych, his closeness to Russia, and the power he wields.
The violence caused political fallout in the President's own party and elsewhere.
Kiev Mayor Volodymyr Makeenko announced his resignation from the country's ruling party, according to the city administration website. The same post also announced that the city's metro transit system is reopening. Government officials had closed the system to prevent protesters from reaching Independence Square, also known as Maidan.
At the Sochi Olympics, Ukrainian athletes wearing black mourning bands held a moment of silence Thursday for fellow citizens slain in the violence erupting in Kiev, the Ukrainian Olympic committee said.
Thursday's violent developments came just hours after Yanukovych announced a truce -- and opposition leaders agreed to abide by it.
Opposition leader Vitali Klitschko, a former world-class boxer, met with Yanukovych Wednesday -- discussions that led to the truce.
They were expected to meet again Thursday, but it was unclear if that meeting would still happen after the most recent violence.
Senior officials in U.S. President Barack Obama's administration told CNN Wednesday they were bracing for Ukraine to intensify its crackdown under pressure from Russia.
"Things have gotten very bad," one official said. "The government is speaking in very nasty, aggressive and confrontational terms. It signals they are prepared to do something."
Analysts warned there was little that outside pressure could do, especially if the Ukrainian military gets involved
on the side of the government cracking down on protesters.
"My own hunch," said Council on Foreign Relations President Richard Haass, "is this is going to continue to escalate."
CNN's Nick Paton Walsh, Phil Black, Andrew Carey and Todd Baxter reported from Kiev, while CNN's Michael Pearson reported and wrote from Atlanta.
™ & © 2014 Cable News Network, Inc., a Time Warner Company. All rights reserved.