KIEV, Ukraine — The fate of Ukrainian President Viktor Yanukovych slipped further into uncertainty on Thursday after dozens of protesters were killed by sniper fire in the bloodiest day of clashes in his country's post-Soviet history prompted Europe-wide sanctions, rebellion in parliament, and a mass exodus of his supporters in the political and financial elite.

With Kiev's central square, the Maidan, covered in flames, smoke, soot, blood, and dead bodies, Yanukovych negotiated well into the night with Ukraine's opposition leaders under the supervision of the French, German, and Polish foreign ministers. A representative from Russia, Vladimir Lukin, Vladimir Putin's human rights ombsudsman, was also at the talks.

Poland's prime minister and the U.S. Ambassador to Ukraine Geoff Pyatt said that Yanukovych expressed a willingness to hold presidential elections, which are currently scheduled for a year from now, before the end of this year. The ministers and Vitaly Klitschko, a boxing champion turned opposition politician, denied a deal had been reached, but said they hoped to have results by Friday.

Yanukovych began to see defections from his Party of Regions, including the government-appointed mayor of Kiev, Vladimir Makeenko. Reports of oligarchs and businessmen fleeing the capital spread on social media.

The fighting began after armed protesters shattered a truce agreed late Wednesday by attacking police and reclaiming Kiev's central square.

Protesters who had held out against police on Independence Square, known as the Maidan, launched an offensive at 7:30 a.m. local time, shooting at police and throwing Molotov cocktails, eyewitnesses said. Fighting then escalated dramatically over the next hour. Riot police returned fire and threw stun grenades before retreating.

Clashes spread from the square to Hrushevskogo Street, the main locus of violence Tuesday where at least 26 died, and up Institutska Street toward a district housing many government buildings. By midmorning, protesters had reclaimed the positions they surrendered when police descended on them on Tuesday.

Police returned fire by sniper and automatic rifle long after the protesters' attack had ceased, however. Activists claimed police snipers shot at men from the square throughout the afternoon. Several of them searched the Ukraina Hotel, which became the main makeshift field hospital on the square, well into the evening in search of the police sniper.

Ukraine's Interior Ministry initially accused the protesters of disguising themselves as police and shooting its officers, 13 of whom died Thursday. Later in the day, however, the ministry admitted that police had been armed throughout the clashes.

Ukraine's health ministry confirmed the deaths of 75 protesters since the start of the week. The real total is expected to be far higher, based on bodies BuzzFeed saw on the square, eyewitness accounts, and estimates by opposition leaders.

Alarmed by the violence, the European Union agreed to issue travel bans and asset freezes against those deemed responsible for the violence. Visiting foreign ministers said a deal to end the crisis had yet to be reached. The men witnessed some of the violence on Thursday, forced to reschedule a meeting as clashes spread.