A deal designed to end Ukraine’s long-running crisis was signed Friday afternoon by President Viktor Yanukovych and the three leaders of the political opposition, but the accord appeared likely to be a hard sell among the thousands of demonstrators who have sustained months of protest.

Violent clashes between hard-line protesters and police erupted Tuesday in Kiev, the Ukrainian capital, after more than a week of relative calm, leaving at least nine people dead and many more wounded.

Violent clashes between hard-line protesters and police erupted Tuesday in Kiev, the Ukrainian capital, after more than a week of relative calm, leaving at least nine people dead and many more wounded.

Deadly clashes between protesters and police in Kiev on Tuesday led to a fire-lit nighttime assault by Interior Ministry troops on the main protest encampment at Independence Square, in what may be a dramatic and irreversible turn in Ukraine’s months-long political crisis.

The 28-nation European Union on Wednesday called a meeting of foreign ministers to decide on its response, including possible sanctions, the Associated Press reported. Sanctions could include travel bans targeting the Ukrainian leadership and asset freezes.

E.U. foreign policy chief Catherine Ashton’s office said the special meeting of foreign ministers would weigh the bloc’s options Thursday in Brussels, according to the AP.

After weeks of relative calm, trucks and tents burned, molotov cocktails smashed against police shields and banners illuminated by the flames whipped in the strong breeze. At least 25 people were reported killed and 240 injured in the latest flare-up of protests that began last fall after President Viktor Yanukovych rejected a trade deal with Europe and turned to Russia for financial help.

Police began to push toward the camp early Wednesday. But whether or not they clear the square, Ukraine is heading for an even deeper divide. The hostility that the opposition feels toward Yanukovych is intense and widespread, especially in the western part of the country.

Having turned to Russia for much-needed financial help, Yanukovych may finally have burned his bridges to the West with Tuesday’s developments, leaving him in danger of being a weakened and unpopular supplicant to Moscow.

The eruption of violence came after nearly a month in which Yanukovych and opposition political leaders warily maneuvered over a new constitution. But early Tuesday afternoon, the parliament, or Verkhovna Rada, refused to take up the issue, and what had been a peaceful demonstration quickly turned into three simultaneous street battles.

The leaders of the protest denounced Yanukovych as the assault on the square, also known as the Maidan, began about 8 p.m. local time. They said he had never intended to reach a deal and had used the weeks of talks to prepare a huge police attack.

Yanukovych’s spokeswoman, Hanna Herman, told Radio Liberty’s Ukrainian service that there would be no further negotiations until the violence stops.

“Negotiations will only take place when the violent methods stop, when the opposition gets its armed people off the street and when calm comes back to the country,” she said. “Then it will be necessary to sit at the negotiating table.”

Lesya Orobets, an opposition member of parliament, said the protesters fell into a trap laid for them by Yanukovych. She said he had knowingly provoked the hard-line members of a right-wing group called Pravy Sektor, who have formed the most aggressive element of the opposition and who led the fighting when it erupted.

“This massacre has been carefully planned in advance and is intended to eventually destroy any hint of democracy in Ukraine,” she wrote on Facebook.

The protests began Nov. 21 when Yanukovych backed away from a trade deal with the European Union, eventually turning to Russia for $15 billion in support. He can now be assured of European hostility.

The United States condemned the explosion of street violence in Ukraine and said the government bears primary responsibility for restoring calm.

Vice President Biden telephoned Yanukovich to express what the White House called “grave concern” and urged the embattled leader to pull back government forces and immediately resume political discussions with opponents.

Biden “made clear that the United States condemns violence by any side, but that the government bears special responsibility to de-escalate the situation,” a White House statement said.

Earlier, White House press secretary Jay Carney said the Obama administration is “appalled” by the violent crackdown on anti-government protesters in the Ukrainian capital.

Washington announced no specific new action, but the U.S. ambassador to Ukraine, Geoffrey Pyatt, threatened both sides with sanctions.

“We believe Ukraine’s crisis can still be solved via dialogue, but those on both sides who fuel violence will open themselves to sanctions,” Pyatt said on Twitter, in both English and Russian.

In Russia, the head of the foreign relations committee of the parliament, Alexei Pushkov, tweeted that Western pressure on Ukraine had “opened the way for radicals.”

The Interior Ministry brought water cannons and armored personnel carriers to the edges of the Maidan. The subway was shut down, and authorities said they were closing off road access to Kiev. In months past, caravans of vehicles, especially from western Ukrainian cities, have often flocked to the capital at times of perceived threats. The country’s leading independent television company, Channel 5, went off the air during the evening in much of Ukraine, according to reports.

Inside their own lines, demonstrators sang the Ukrainian national anthem.

“We will not fall for their provocation, but we won’t retreat even one step from here, from this Maidan, and we don’t have anywhere to retreat to,” an opposition leader, Arseniy Yatsenyuk, told the crowd during the evening.

“We remain here; we are defending our Maidan, we are defending our Ukraine, and we are defending our future,” Yatsenyuk said.

The Interior Ministry said that at least 13 civilians and seven police officers were killed. There were widely circulated reports of additional deaths.

Early in the evening, Interior Ministry troops and hired civilians — popularly known as “titushki” — could be seen on Web video streams moving down Hru­shevsky Street, where violent clashes occurred in January. They stormed Ukraine House, an exhibition center on nearby European Square that had been taken by protesters in January.

Then they turned and moved toward the Maidan.

The fighting had begun in the streets around the parliament, which once had been in the firm control of the police.

Snipers were reported on rooftops. At one point they were confronted on a roof by protesters carrying steel rods, according to witnesses. The snipers withdrew.

Opposition forces stormed the headquarters of the ruling Party of Regions, though it was later retaken by government supporters who said they found the body of an office worker inside.

Several Interior troops were captured by demonstrators, according to reports, and taken to the Maidan as prisoners.

The shift toward violence was abrupt. In recent weeks, an amnesty had led to the release of nearly all those arrested over the winter in connection with the protests, and opposition forces had abandoned the city hall and partially pulled back from Hrushevsky Street.

Parliament, controlled by the Party of Regions, was to consider constitutional changes that would give Ukraine a governing system with a strong parliament and weak president.

But opposition political leaders showed very little trust in Yanukovych even before Tuesday, and their more militant followers have proved difficult to control.

Tuesday’s violence led Vitali Klitschko, head of the opposition UDAR party, to declare that Yanukovych must agree to early elections for president and parliament. The next scheduled presidential election is in 2015.

Ashton, the top diplomat for the European Union, said in a statement: “I am deeply worried about the grave new escalation in Kiev and the reported victims. I condemn all use of violence, including against public or party buildings.”

The opposition said more than 100 protesters had been injured by police.

“Soldiers, don’t take blood onto your hands by protecting these gangsters in power,” Yuri Lutsenko, once the interior minister and now a protest leader, said on the stage at the Maidan, according to the Kyiv Post. “If you set foot on the Maidan, this is your choice. Whoever passes this threshold determines their country’s future.”

He added: “You won’t be a traitor if you join us. Show your true soul and hearts.”

Anne Gearan contributed to this report.