KIEV, Ukraine (CNN) — A violent showdown appeared likely Tuesday night in Ukraine after new clashes left 14 people dead and the capital’s square afire amid monthslong turmoil over whether the country should ally itself economically with Russia or Europe, officials said.

With 47 police officers injured in clashes with protesters favoring a European Union pact, ominous warnings were issued by Kiev officials and the U.S. Embassy for Tuesday night. At least 14 people died in protests, including six police officers, authorities said.

City Hall officials warned residents to stay away from the city center “to avoid casualties,” and the U.S. Embassy said Ukrainian security services “may take extraordinary measures” following the fiery violence.

The U.S. Embassy “advises all citizens to maintain a low profile and to remain indoors tonight,” the mission said Tuesday.

Protesters set parts of the city center afire Tuesday night by burning tires, sending black smoke through Kiev.

Explosions ruptured the night air, but it was unclear whether they emanated from protesters’ firework or a police action. Protesters fed several fires by tossing wood upon the bonfires in Independence Square, which was under the demonstrators’ control. They also built barricades against a potential police offensive.

Meanwhile, Arseniy Yatsenyuk, one of the opposition leaders, made a public appeal to President Viktor Yanukovych: “Do not let Ukraine become a country covered with blood. Pull back the police and announce a cease-fire. Then we will negotiate.”

Stefan Fule, the European Union’s commissioner for enlargement and European neighborhood policy, expressed alarm at the violence.

“To learn that there have been deaths and hundreds if injured people makes me shake. I deplore deaths of those people and my thoughts are with their families,” Fule said.

“I was just on the phone with the acting Prime Minister (Serhiy) Arbuzov telling him that seeing in the streets Berkut, the special police, with Kalashnikov(s) is source of great concern,” Fule added. “He assured me that he and the government would do what they can to make sure that those weapons stay silent. For the sake of Ukraine and its future I will pray that he is right.”

U.S. Ambassador Geoffrey Pyatt said he hoped the government and opposition will take steps to reduce the escalation of violence “and the return to political dialogue.”

“We want to see a Ukraine that is stable and democratic,” Pyatt said.

As violence roiled, the Interior Ministry said metro stations were closed.

The protesters’ medical service said more than 150 people were hurt in Tuesday’s clashes.

Thousands of demonstrators have packed Kiev’s Independence Square since November, when Yanukovych reversed a decision to sign a trade deal with the European Union and instead turned toward Russia.

The unrest intensified after an anti-protest law went into effect. Throngs of demonstrators took to the streets to protest the law.

More than 2,000 people have been injured during the clashes.

Portrait of violence

As temperatures hovered around freezing about 10:30 p.m. (3:30 p.m. ET), Independence Square was the central theater for the protest, as it has been, in some sense, for the past three months since it was first occupied and turned into a large fortified camp flush with barricades.

After a day of clashes, protesters braced for another onslaught by riot police Tuesday night.

Authorities had vowed to clear the central Kiev square earlier in the day, moving in with force that proved fatal for protesters and police alike.

Video from earlier Tuesday showed riot police forcing their way through protesters, many carrying shields and swinging sticks. A few protesters responded by swinging bats themselves, all of it contributing to a violent, chaotic melee in the center of Ukraine’s capital.

The situation hardly calmed as night descended in Kiev. Fires raged around downtown, with small explosions regularly erupting — the product of protesters’ fireworks and, perhaps, the stun grenades that police have been using to clear the crowds.

Authorities have also used water cannons and restricted traffic into Kiev as part of their effort to clamp down as part of what it called “anti-terror” operations.

“The truce has been broken,” said Viktor Pshonka, Ukraine’s prosecutor general, in blaming the opposition for the rising violence and rationalizing the stepped-up response. “For the sake of pursuing their own political interests, they neglected all previously reached agreements and put lives and the peace of millions of Kiev residents under threat.”

On Tuesday, the casualties occurred after protesters set fire to the headquarters of the ruling Party of Regions and as violence roiled the capital for the first time in more than two weeks.

The opposition confirmed five of the deaths — three near parliament and two near a metro station; the Interior Ministry said a policeman died in an ambulance en route to a hospital.

Earlier, an opposition member of Ukraine’s parliament told CNN that three protesters had died and seven others were seriously injured during protests Tuesday in Kiev.

Speaking from the protesters’ medical facility outside the parliament building, Olesya Orobets said ambulances had been barred from the area.

The prosecutor general, who said in a statement that at least 100 people had been hurt, blamed the protesters for the violence.

“Today, we were able to see that only the government is interested in peaceful resolution of the situation,” Pshonka said. “Opposition leaders should take the responsibility for everything happening in the street of Kiev today. It is the opposition who announced a peaceful rally that turned into violent standoff.”

He vowed to hold the organizers accountable “for every single person injured, every car burned and every window broken.”

United Nations Secretary-General Ban Ki-moon took note of renewed efforts in the Ukraine for meaningful dialogue but expressed concern about the violence and appealed to all participants to act with restraint, his spokesman said.

In Britain, Foreign Office Minister for Europe David Lidington said in a statement that he was “appalled” by the reports. “This has no place in a European democracy,” he said. “I condemn the violence and urge all sides to immediately de-escalate the situation.”

He called on Ukraine to use a return to stability to tackle the protests’ underlying causes — “corruption, impunity, and the lack of checks and balances within the current governmental system.”

In a statement, NATO Secretary General Anders Fogh Rasmussen called on all parties “to refrain from violence and to urgently resume dialogue, including through the parliamentary process.”

A sliver of hope for a restoration of calm had emerged Sunday, when the government agreed to drop charges against demonstrators in exchange for the protesters’ agreement to leave Kiev’s City Hall and unblock streets in the city center, which they had occupied for almost three months.

But violence erupted again Tuesday, after the speaker of the parliament would not let opposition members register amendments that would have led to a vote to limit the rights of the President and restore the constitution to the way it was in 2004.