KIEV (CNN) - Ukraine's health ministry reported Tuesday that 25 have died in clashes between protesters and government forces.
Another 241 were hospitalized.
At least seven of the dead were police officers. As violence continued, the ministry said metro stations were closed, and Kiev officials urged residents to stay away from the city center to avoid danger.
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."
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.
Speaking from the protesters' medical facility outside the parliament building, Olesya Orobets said ambulances had been barred from the area.
The prosecutor general blamed the protesters for the violence.
"Today we were able to see that only the government is interested in peaceful resolution of the situation," Viktor 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 spoke out about the bloodshed, calling it unacceptable. He urged the parties to return to the negotiation table.
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."
Thousands of demonstrators have packed Kiev's Independence Square since November, when President 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.
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.
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